For all STAR physics focus articles in list form, click HERE

STAR focus: Measurement of sequential Upsilon suppression in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with the STAR experiment

A primary goal of the RHIC physics program is to study the properties of the Quark-Gluon Plasma (QGP), a novel state of QCD matter consisting of deconfined quarks and gluons. Among various probes used, Υ mesons, bound states of bottom and anti-bottom quark pairs, play a unique role as they are believed to be produced before the QGP formation, and then get destroyed in the QGP due to the color-screening of the potential between the bottom and anti-bottom quarks as well as scatterings with medium constituents. There are three Υ states (Υ(1S), Υ(2S), Υ(3S)), which possess different amounts of binding energies, with Υ(1S) being bounded the strongest while Υ(3S) the weakest. Consequently, they are expected to experience different levels of yield suppression in the QGP depending on the interplay between the medium temperature and their binding energies. Measurement of such sequential suppression for the three Y states can be used to study the modification of the QCD force in the medium and the QGP’s thermodynamic properties.

Recently, the STAR experiment published the first measurement of yield suppression for the three Y states separately in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, as shown in the figure below (PRL 130 (2023) 112301). The level of suppression is quantified using the nuclear modification factor (RAA), a ratio between the Y yield in Au+Au collisions to that in p+p collisions scaled properly to account for the trivial geometric difference between nucleon+nucleon and nucleus+nucleus collisions. In the 0-60% centrality class, RAA for all three Y states are much smaller than 1, indicating large losses of yields in the QGP. It is worth noting that a large fraction of the measured Y(1S) suppression arises from the decreased Y(2S) and Y(3S) yields that contribute to the Y(1S) production through decays. Comparing among different Y states, Y(3S) is significantly more suppressed than Y(1S) while Y(2S) lies in between. The result is consistent with the expected sequential suppression for the Y family in heavy-ion collisions, and can be used to provide additional constraints to model calculations, a necessary step to infer QGP properties from these measurements. With the large sample of Au+Au collisions to be collected by the STAR experiment in 2023 and 2025, a significant improvement in the measurement precision, especially for the excited Y(2S) and Y(3S) states, is foreseen.


Figure: Left: Y(1S) (circles) and Y(2S) (squares) RAA as a function of Npart for pT < 10 GeV/c. Data points for Y(2S) are displaced horizontally for better visibility. The vertical bars on data points indicate statistical errors, while the systematic uncertainties are shown as boxes. Shadowed bands around each marker depict the systematic uncertainties from Ncoll . The bands at unity indicate the global uncertainties. Right: RAA for various Y states, including the 95% upper limit for Y(3S), in 0-60% Au+Au collisions.

Posted Mar 15, 2023

STAR focus: Evidence of Mass Ordering of Charm and Bottom Quark Energy Loss in Au+Au Collisions at RHIC

Studying the properties of the Quark Gluon Plasma (QGP) created in heavy-ion collisions is a main goal of the RHIC physics program. Heavy quarks, i.e., charm and bottom quarks, have emerged as essential probes of the QGP as they are produced predominantly at the initial stage of the heavy-ion collisions and subsequently experience the entire evolution of the QGP. In particular, heavy quarks are expected lose energy in the QGP via (quasi-)elastic scatterings with the medium constituents and induced gluon radiation, and QCD predicts that heavy quarks lose less energy than light quarks due to the so-called “dead cone” effect. Therefore, parton energy loss in the QGP is expected to follow a hierarchy ordered by parton color charge and mass. Experimentally, nuclear modification factors, RAA and RCP, are measured to study the parton energy loss.

Recently, the STAR Collaboration published "Evidence of Mass Ordering of Charm and Bottom Quark Energy Loss in Au+Au Collisions at RHIC" in Eur. Phys. J. C 82 (2022)1150. In this publication, the new measurements of RAA for inclusive heavy flavor-decay electrons and separately for bottom- and charm-decay electrons, as well as double ratios of bottom- and charm-decay electron RAA and RCP in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV are reported. We find the bottom-decay electron RAA and RCP to be significantly higher than those of charm-decay electrons. Duke and PHSD model calculations, including mass-dependent parton energy loss in a strongly coupled medium, are found to be consistent with data. These observations provide strong evidence of the mass ordering of charm and bottom quark energy loss in the QGP.


Figure: (a) The RAA ratio of bottom- to charm-decay electrons in intervals of electron pT in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. (b) The RCP ratios of bottom-decay electrons to that of charm-decay electrons in intervals of electron pT in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. The red diamonds show the ratios of RCP (0−20%/40−80%), and the blue circles show the ratios of RCP (0−20%/20−40%). In all panels the error bars and the brackets show statistical and systematic uncertainties, respectively. The Duke and PHSD models are shown as the various lines. The null hypothesis calculations are shown as the shaded bands.

Posted Mar 8, 2023

STAR focus: Higher-order cumulants and correlation functions of proton multiplicity distributions in $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV Au+Au collisions at the RHIC STAR experiment

Experimental observations at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have confirmed the existence of Quark-Gluon Plasma (QGP) in high-energy heavy-ion collisions at μB ~ 0. The transition from hadronic matter to QGP is believed to be a crossover based on Lattice QCD calculations, but there may be a first-order phase transition and a critical point at finite baryon chemical potential in the QCD phase diagram. Higher-order cumulants of conserved quantities (Baryon number, Electric charge, and Strangeness) have been suggested as sensitive observables to search for the critical point and phase boundary in the QCD phase diagram. The fourth-order cumulant ratio (C4/C2) of conserved quantities is predicted to exhibit a non-monotonic energy dependence due the critical point. A recent paper on net-proton cumulants from the RHIC beam energy scan program has demonstrated a non-monotonic trend, but with large statistical uncertainty.


Figure: Collision energy dependence of the ratios of cumulants, C4/C2, for proton (squares) and net-proton (red circles) from top 5% Au+Au collisions at RHIC. The points for protons are shifted horizontally for clarity. The new result for proton from the $\sqrt{s_{\rm NN}}$ = 3 GeV collisions is shown as a cyan filled square. HADES data of 2.4 GeV top 10% collisions is also shown. The vertical black and gray bars are the statistical and systematic uncertainties, respectively. In addition, results from the transport model UrQMD are presented.

As a longer version of the recent published paper Phys. Rev. Lett. 128, 202303, the paper Phys. Rev. C 107, 024908 [Editors’ Suggestion] summarizes in detail the analysis of proton cumulant and correlation function measurements of STAR fixed-target $\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collision data. The paper discusses the effect of pileup which is important in fixed-target data analysis and gives a correction method in higher-moments measurements. In addition, at low collision energy, due to limited reference multiplicity to define centrality, a large initial volume fluctuation effect is seen. A model dependent method to suppress the volume fluctuation effect is tested in this work and shows that the most central centrality is least affected by the effect. However the method is not ideal due to its model dependency.

The above figure shows the net-proton and proton fourth-order cumulant ratio C4/C2 from the top 5% most central Au+Au collisions. As one can see there is a clear non-monotonic energy dependence in the data but a monotonic dependence in the model calculation. The cyan filled square at $\sqrt{s_{\rm NN}}$ = 3 GeV represents the new measurement. The consistency of data with UrQMD calculation (yellow cross) implies that the collisions occurred at 3 GeV are dominated by hadronic interactions. It is expected that with more data from the RHIC beam energy scan phase II, the uncertainty will be reduced.

Posted Feb 23, 2023

STAR focus: Pion, kaon, and (anti-)proton production in U+U Collisions at $\sqrt{s_{\mathrm{NN}}}$ = 193 GeV measured with the STAR detector

In general, attempts in relativistic heavy-ion collisions have been made to achieve maximum energy densities in the collisions. One way to do so is by utilizing Uranium+Uranium collisions. Because of the prolate deformation of the nuclei, U+U collisions at the same beam energy and impact parameter but different orientations are expected to form dense matter with varying compressions and lifetimes. Specifically, the deformation of uranium nuclei allows for a gain in particle multiplicity (models suggest 6% to 35% more than Au+Au collisions) and energy density by aligning the two nuclei with their long axes head-on (tip-tip). Further, the prolate deformed uranium geometry allows for an initial temperature distribution that also depends on the relative spatial orientation of the two nuclei. Some of these nuclei will collide along their long axes, creating denser matter than routinely created at RHIC in collisions of gold nuclei, which are more spherical. Some nuclei will collide with their long axes parallel, although perpendicular to their directions of motion. This arrangement creates a matter with an oblong cross-section but without the strong magnetic field generated by grazing incidence collisions of spherical nuclei. All these have consequences on observables like particle yields, elliptic flow, nuclear modification factors and correlations. In the year 2012, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) collided the beams of uranium ions (the heaviest ions ever used in a collider). The STAR experiment took data for Uranium on Uranium (U+U) collisions at $\sqrt{s_{\mathrm{NN}}}$ = 193 GeV, allowing for collecting data for one of the largest numbers of participating nucleon systems.

In this work, the STAR Collaboration has performed some basic experimental measurements and studied the bulk properties of the medium in U+U collisions at $\sqrt{s_{\mathrm{NN}}}$ = 193 GeV averaged over all orientations of the colliding nuclei. The identified particle ($\pi$±, $K$±, $p$, and $\bar{p}$) results on transverse momentum spectra, the particle yields (dN/dy), the mean transverse momentum (⟨pT⟩), the particle ratios, and the kinetic freeze-out parameters are obtained and compared with the corresponding published results of Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. A comparative study with AMPT model modified to incorporate the deformation of uranium nucleus is also carried out. These results have been recently published in Physical Review C.

/ Figure: dN/dy of $π$+, $K$+, $p$ and $\bar{p}$ scaled by ⟨Npart⟩/2 as a function of ⟨Npart⟩ at midrapidity (|y| < 0.1) for U+U collisions at $\sqrt{s_{\mathrm{NN}}}$ = 193 GeV. The results are compared with Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. The uncertainties represent total systematic and statistical uncertainties added in quadrature, dominated by systematic uncertainties.

The experimentally measured value of the average number of participating nucleons obtained in U+U is greater than 400, which is higher than that in Au+Au collisions. Comparison of results from U+U with those from Au+Au collisions as a function of the number of participating nucleons shows that for a given number of participating nucleons value, the results are consistent between U+U and Au+Au collisions. These suggest that combining all different initial state orientations of the colliding uranium nuclei give similar results as one would obtain by colliding spherically symmetric nuclei. These results depend only on the number of participating nucleons but not on different systems. The AMPT model, with three different nucleon cross-sections studied, does not consistently describe all the observables at all numbers of participating nucleon values. These findings call for further investigation into the AMPT model in terms of the particle production mechanism and the collision dynamics.

In future experiments involving colliding uranium nuclei, it would be interesting to measure the different final state observables for identified orientations of the colliding ellipsoidal uranium nuclei.

Posted Feb 9, 2023

STAR focus: Pattern of Global Spin Alignment of φ and K*0 mesons in Heavy-Ion Collisions

The STAR Collaboration recently published the “Pattern of Global Spin Alignment of φ and K*0 mesons in Heavy- Ion Collisions” in Nature.

Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely φ and K*0, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for φ is unexpectedly large, while that for K*0 is consistent with zero. The ρ00 for φ mesons, averaged over beam energies between 11.5 and 62 GeV is 0.3512 ± 0.0017 (stat.) ± 0.0017 (syst.). Taking the total uncertainty as the sum in quadrature of statistical and systematic uncertainties, our results indicate that the φ-meson ρ00 is above 1/3 with a significance of 7.4σ. Th ρ00 for K*0, averaged over beam energies of 54.4 GeV and below is 0.3356 ± 0.0034 (stat.) ± 0.0043 (syst.).


Figure: Global spin alignment of φ and K*0 vector mesons in heavy-ion collisions. The measured matrix element ρ00 as a function of beam energy for the φ and K*0 vector mesons within the indicated windows of centrality, transverse momentum (pT ) and rapidity (y). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV. The red solid curve is a fit to data in the range of $\sqrt{s_{\mathrm{NN}}}$ = 19.6 to 200 GeV, based on a theoretical calculation with a φ-meson field. Th red dashed line is an extension of the solid curve with the fitted parameter Gs(y). The black dashed line represents ρ00 = 1/3.

Measurements of the global spin alignment of vector mesons is argued to provide new knowledge about the vector meson fields. The vector meson fields are an essential part of the nuclear force that binds nucleons inside atomic nuclei, and are also pivotal in describing properties of nuclear structure and nuclear matter. The ρ00 for the φ meson has a desirable feature in that all contributions depend on squares of field amplitudes; it can be regarded as a field analyzer which makes it possible to extract the imprint of the φ-meson field even if the field fluctuates strongly in space-time. Another important feature worthy of mention is that an essential contribution to the φ-meson ρ00 is from the term $~S\cdot(\mathrm{E}_{\phi}\times \mathrm{p})$, where Eφ is the electric part of the φ-meson field induced by the local, net strangenes current density, and S and p are the spin and momentum of the strange (anti)quarks, respectively. Such a term is nothing but the quark version of the spin-orbit force which, at the nucleon level, plays a key role in the nuclear shell structure. Our measurements of a signal based on global spin alignment for vector mesons reveal a surprising pattern and a value for φ meson that is orders of magnitude larger than can be explained by conventional effects. This work provides a potential new avenue for understanding the strong interaction at work at the sub-nucleon level.

Posted January 19, 2023

STAR focus: Azimuthal transverse single-spin asymmetries of inclusive jets and identified hadrons within jets from polarized pp collisions at √s = 200 GeV

Recently, the STAR Collaboration published the most precise measurement of the transverse single-spin asymmetries for charged hadrons inside jets in Phys. Rev. D 106, 072010.

Transverse spin experiments at STAR provide new ways to map out the three-dimensional nature of quark fragmentation and illustrate the interplay between the structure of a hadron and the color environment. The Collins effect describes the azimuthal distribution of hadron fragments from a transversely polarized quark, and thus provides access to both the quark transversity in the proton and the transverse momentum dependent Collins fragmentation function. Transversity is one of the three leading twist parton distribution functions of the nucleon. It describes the transverse spin structure of quarks in a transversely polarized proton. The Collins fragmentation function is one of the most important fragmentation functions in the transverse-momentum-dependent (TMD) formalism. This effect has only been studied in semi-inclusive deep-inelastic scattering (SIDIS) and electron-positron annihilation before the STAR experiment.


The figure presented above shows the first measurement of Collins asymmetry with jT (momentum transverse to the jet axis) dependence in pp collisions at √s = 200 GeV. The results here are divided into six different jet-pT bins that probe different hard scales in the experiment. The DMP+2013 and KPRY model expectations are also presented in the plot. Both are based on fits to experimental data from SIDIS and electron-positron processes. Our results slightly favor the KPRY model that treats TMD evolution up to the next-to-leading logarithmic effects; however, significant discrepancies exist between the data and both model calculations. These new results will provide valuable new constraints on the kinematic dependence of the Collins fragmentation function when included in future global analyses.

Posted Oct 28, 2022

STAR focus: Evidence for Nonlinear Gluon Effects in QCD and their A Dependence at STAR

Extracting the data from previous experiments, gluon density grows rapidly towards small momentum fraction (x) with respect to the nucleon. Under the color glass condensate (CGC) framework, the growth is explained by gluon splitting. The nonlinear QCD effects at small x should tame this growth by gluon recombination. The so called “gluon saturation” is reached at the point when the splitting and recombination are balanced. Understanding the nonlinear behavior of the gluon is one of the most important physics goals for RHIC Cold QCD program and the future electron ion collider (EIC) project.

Back-to-back dihadron azimuthal angle correlation has been proposed to be a sensitive probe to directly access the underlying gluon dynamics involved in hard scatterings. With a high gluon density at the initial state, the product of back-to-back dihadron modulation will be suppressed. It is predicted that the density of gluons per unit transverse area is larger in nuclei than in nucleons and is amplified by a factor of A1/3 for a nucleus with mass number A; thus, nuclei provide a natural environment to study nonlinear gluon evolution.

The STAR Collaboration performed the measurements of back-to-back azimuthal correlations of di-π0s produced at forward pseudorapidities (2.6 < η < 4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass energy of 200 GeV. The results have been recently published in Phys. Rev. Lett. 129, 092501. We observe a clear suppression of the correlated yields of back-to-back π0 pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression is larger at smaller transverse momentum, which indicates lower x and Q2 . The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale Q2s on the mass number A. A linear scaling of the suppression with A1/3 is observed with a slope of −0.09±01.


Figure: Relative area of back-to-back di-π0 correlations at forward pseudorapidities (2.6 < η < 4.0) in p+Au and p+Al with respect to p+p collisions for $p_T^{\mathrm{trig}}$=1.5–2 GeV/c and $p_T^{\mathrm{asso}}$=1–1.5 GeV/c. The area is the integral of the back-to-back correlation after pedestal subtraction. The data points are fitted by a linear function, whose slope (P) is found to be −0.09±01.

Posted August 22, 2022

STAR focus: Measurements of proton high order cumulants in $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV Au+Au collisions and implications for the QCD critical point

With the discovery of the quark-gluon plasma (QGP) at the Relativistic Heavy Ion Collider (RHIC), physicists are starting to investigate the phase structure of the QCD matter, especially in the high baryon density region. The stark differences between the properties of QGP and lower energy nuclear matter draw interest to the thermodynamic processes, specifically those related to the nature of phase transitions. Experimenters can access the QCD phase diagram, expressed in temperature (T) and baryonic chemical potential (μB), and search for phase boundaries by varying the heavy-ion collision energy. At regions of equal baryon and anti-baryon density, μB = 0, theoretical approaches work well, with lattice QCD calculations predicting a smooth cross-over transition from hadronic matter to a QGP. In addition, Lattice QCD calculations have predicted a positive cumulant ratio of net-proton (proton minus anti-proton) C4/C2 for the formation of QGP matter at μB = 200 MeV [1]. However, at finite μB, the existence and the nature of the phase transition are not well understood.

Recent reports on net-proton fluctuation measurements from RHIC's Beam Energy Scan program (BES-I) have demonstrated the non-monotonic collision energy dependence in the 4TH order net-proton cumulant ratio C4/C2 from the top 5% central Au+Au collisions of the range of 7.7 – 200 GeV [2].


Figure: Collision energy dependence of the ratios of cumulants, C4 /C2 , for proton (squares) and net-proton (red circles) from top 5% Au+Au collisions at RHIC. The points for protons are shifted horizontally for clarity. The new result for proton from the $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV collisions is shown as a filled square. HADES data of 2.4 GeV top 10% collisions is also shown. The vertical black and gray bars are the statistical and systematic uncertainties, respectively. In addition, results from the HRG model, based on both Canonical Ensemble (CE) and Grand-Canonical Ensemble (GCE), and transport model UrQMD are presented.

In this Letter [3], we report cumulants and their ratios of proton multiplicity distribution from $\sqrt{s_{\mathrm{NN}}}$ =3 GeV Au+Au collisions. The new data are measured by the STAR experiment configured in fixed-target mode. At this collision energy, the corresponding baryonic chemical potential μB ~ 750 MeV, close to the largest value ever reached in heavy ion collisions. Protons are measured with the acceptance (-0.5 < y < 0 and 0.4 < pT < 2.0 GeV/c). The rapidity and transverse momentum dependencies of the cumulant ratios C2/C1, C3/C2 , and C4/C2 are discussed. As shown in the figure, a suppression with respect to the Poisson baseline is observed in proton C4/C2 = -0.85 0.09 (stat) 0.82 (syst) in the most central collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV. The hadronic transport model UrQMD reproduces the observed trend in the centrality dependence of the cumulant ratios. This new result is consistent with fluctuations driven by baryon number conservation at the high baryon density region. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV.

[1] A. Bazavov et al. (HotQCD), Phys. Rev. D96, 074510 (2017) and R. Bellwied et al., Phys.
Rev. D104, 094508 (2021).
[2] (STAR Collaboration), Phys. Rev. Lett. 126 (2021) 92301.
[3] (STAR Collaboration), Phys. Rev. Lett. 128, (2022) 202303.

Posted May 29, 2022

STAR focus: Measurements of 3ΛH and 4ΛH lifetimes and yields in Au+Au collisions in the high baryon density region

The hyperon-nucleon (Y-N) interaction is an important ingredient in the description of the equation-of-state of high baryon density matter, such as the interior of neutron stars and the hadronic phase of a heavy-ion collision. Hypernuclei, being bound states of hyperons and nucleons, are one of the only means for experimentalists to access to the Y-N interaction. However, hypernuclei meausurements in heavy-ion collisions are scarce, mainly due to the low production rates at high energies. In contrast, at low collision energies, an enhancement in the hypernuclei production yield is expected due to the higher baryon density, although this has not been verified experimentally. The STAR collaboration recently published new measurements in Phys. Rev. Lett. 128, 202301 of the yields of two light hypernuclei (3ΛH and 4ΛH) at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV, together with measurements of their lifetimes, using the Beam-Energy-Scan-II data taken in 2018.


Figure (a) 3ΛH and (b) 4ΛH yields at mid-rapidity as a function of beam energy in central heavy ion collisions. The symbols represent measurements while the lines represent different theoretical calculations. The data points assume a branching ratio of 25(50)% for 3(4)ΛH → π + 3(4)He. The insets show the yields at |y|<0.5 times the branching ratio as a function of the branching ratio.

Our measurements show that in central heavy-ion collisions, the 3ΛH yield at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV is enhanced compared to the yield at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV by approximately a factor of 100, in accordance with thermal model and coalescence model predictions. For 4ΛH, the measured yield is underestimated by thermal model calculations. Such observations establish low energy collision experiments as a promising tool to study hypernuclei properties, and also provide guidance on searches for exotic strange matter such as double-Λ hypernuclei or strange dibaryons.

Besides the production yields, the intrinsic properties of hypernuclei, such as their binding energies and lifetimes, provide information on the Y-N interaction. Taking advantage of the high production yields, we extracted the most precise 3ΛH and 4ΛH lifetimes to date. Both lifetimes are shorter compared to the free Λ lifetime by approximately 20%. The 3ΛH lifetime is consistent with theoretical calculations incorporating attractive pion final state interactions, while the 4ΛH lifetime is consistent with estimations based on the emperical isospin rule. Such precise measurements provide means to verify our understanding of the simplest bound Y-N systems.

Posted May 23, 2022

STAR focus: Differential measurements of jet substructure and partonic energy loss in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV

The STAR Collaboration recently published the first “Differential measurements of jet substructure and partonic energy loss in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV" in Phys Rev C 105 (2022) 4, 044906.

Parton energy loss serves as the earliest signature of the Quark-Gluon Plasma (QGP) in central heavy-ion collisions. Hard-scattered partons traverse through the QGP, and through their interactions with the medium, lose energy via many different channels. Thus it is crucial to study the degree to which the energy loss is dependent on the parton shower topology. In this measurement, for the first time, STAR has quantified an angular or resolution scale for a particular jet, and differentially measured the energy loss for two populations, one with wider opening angles and one with narrower angles.

Data from Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV, from 2007, with a single high energy calorimeter tower are selected and both trigger and recoil jets are reconstructed with charged particle tracks and towers with ET > 2 GeV. These di-jets with a hard-constituent selection are termed Hard-Core di-jets, which are primarily free of any combinatorial background contribution. Once we find Hard-Core jets, we drop the 2 GeV threshold down to 0.2 GeV and run the jet finder over all the soft tracks and towers. The di-jets which geometrically match in η-φ space to our Hard-Core di-jets are now called the Matched di-jets. The advantage of such a procedure is to provide us with two collections of di-jets originating from a hard scattering, one with only high momentum particles which undergo energy loss, and the other including both high and low momentum particles, which can potentially include the recovery of the quenched energy. The reference dataset for this measurement is proton-proton collisions also at $\sqrt{s}$ = 200 GeV collected in 2006 and embedded into minimum-bias AuAu collisions from 2007. This embedded reference ensures that the effect of the heavy-ion background and the STAR detector are comparable and any potential differences between the datasets can be attributed to the effects of topology/substructure dependent energy loss.

For the differential measurement, the di-jet pairs are tagged based on the recoil jet’s opening angle defined using the subjets. This is a new substructure observable introduced in this publication which re-clusters the tracks and tower constituents of the anti-kT R=0.4 jet into subjets with the same anti-kT algorithm but a smaller jet radius of R=0.1. The leading and sub-leading subjets are then selected and the η-φ distance between the subjet axes is taken as the opening angle observable θSJ. The figure presented above shows both the di-jet asymmetry for Matched di-jets on left and a cartoon showing the subjet opening angle on the right for the recoil jets. The blue markers represent narrow recoil jets and the red markers include the wide subjets. For both, wide and narrow jet populations, we find that the energy loss experienced by high pT particles is fully recovered within the jet cone included in soft particles as shown by a balanced AJ of Matched di-jets for both Au+Au (solid markers) and the embedded reference (open markers). This shows the first evidence for energy loss being independent on the jet topology, i.e. its opening angle. With such differential measurements, we can now quantify the mechanism of energy loss for specially selected di-jets are due to soft-gluon radiation off a single color charge undergoing the QCD equivalent of the LPM effect.

Posted May 4, 2022

STAR focus: Probing the gluon structure of the deuteron with J/ψ photoproduction in d+Au in ultra-peripheral collisions

In a recent measurement published in Phys. Rev. Lett. 128, 122303, the STAR Collaboration has reported a result on colliding high-energy photons with gluons inside the deuteron. It has provided a first glimpse of the gluonic structure of the simplest atomic nucleus. The momentum distribution of gluons, measured through the J/ψ particle’s momenta shown in the figure, reflects the spatial distribution of gluons inside the nucleus. In addition, the breakup of the deuteron associated with the J/ψ particle probes the gluon dynamics of the nucleon-nucleon interaction, laying the foundation for its precision measurement at the upcoming Electron-Ion Collider.

Figure: Upper: differential cross section as a function of p2T,J/ψ of J/ψ photoproduction in UPCs at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. Data for the total diffractive process are shown with solid markers, while data with neutron tagging in the deuteron-going ZDC are shown with open markers. Theoretical predictions based on the saturation model (CGC) and the nuclear shadowing model (LTA) are compared with data, shown as lines. Statistical uncertainty is represented by the error bars, and the systematic uncertainty is denoted by the shaded box. Lower: ratios of total data and models are presented as a function of −t ≈ p2T,J/ψ. Color bands are statistical uncertainty based on the data only, while systematic uncertainty is indicated by the gray box.

Measuring diffractive Vector-Meson production, e.g. J/ψ, has been one of the most powerful tools in studying the nucleon and nucleus structure in high energy particle collisions. Instead of using a hadron projectile, a high-energy photon emitted by the gold nucleus has been used to probe the inner structure of the deuteron target, with its advantage of being a clean probe. This is type of collision is known as an “Ultra-Peripheral Collision” (UPC). A naïve picture of this process is the following. An incoming photon fluctuates into a quark-antiquark pair and forms a J/ψ particle with close to zero transverse momentum. The gluons jiggling inside the deuteron, although happening very rarely, can kick the J/ψ particle and it would deflect with some momentum. This momentum kick is a Fourier transform of the position of gluons, such that the position and momentum of gluons are the two sides of the same coin. Knowing one side would imply the understanding of the other.

The Zero-Degree-Calorimeter (ZDC), a detector 18 meters away from the center of the STAR main detector, can detect breakup neutrons from the deuteron. Understanding the nuclear breakup has been one of the challenges in Ultra-Peripheral Collisions, as well as at the Electron-Ion Collider. The reported data based on the deuteron has provided an essential experimental baseline on how this system breaks apart and provide quantitative constraints to leading theoretical models.

Posted March 24, 2022

STAR focus: Disappearance of partonic collectivity in $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV Au+Au collisions at RHIC

Determining the nature of phase transition from hadronic matter to the Quark-Gluon Plasma (QGP) phase of the Quantum Chromodynamics (QCD) phase diagram at finite net-baryon density has been the focus of the RHIC beam energy scan program. The directed flow (v1) and elliptic flow (v2) are excellent probes for studying properties of the nuclear matter created in high-energy nuclear collisions owing to their sensitivity to the expansion dynamics. On the other hand, v1 and v2 are particularly sensitive to the Equation-of-State (EoS) and degrees of freedom of nuclear matter. The large positive v2 along with the observation of its number-of-constituent-quarks (NCQ) scaling are strong evidence for the formation of a hydrodynamically expanding QGP phase with partonic degrees of freedom. Flow measurements at high baryon density region, e.g. 3 GeV, provide information that the created nuclear matter is dominant by hadronic or partonic degrees of freedom, thus explore the QCD phase structure.

STAR collaboration recently published a new measurement of v1 and v2 for identified hadrons in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 3, 27, and 54.4 GeV (Phys. Lett. B 827 (2022) 137003). The lowest collision energy that RHIC has reached, 3 GeV, corresponds to a baryon chemical potential $\mu_{B} \sim$ 750 MeV on the QCD phase diagram. The data were taken under the fixed-target configuration with the STAR detector covering the full acceptance from mid-rapidity to the target rapidity region. In this paper, at the two higher collision energies, the observed v2 values at mid-rapidity are positive and consistent with previous measurements. Meanwhile, the NCQ scaling of v2 holds well indicating partonic collectivity has been built-up. Contrary to the results from higher collision energies, the measured v2 values at mid-rapidity for all hadrons are negative and the NCQ scaling disappears for the positively charged particles in 3 GeV Au+Au collisions. In addition, the v1 slopes at mid-rapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. Furthermore, calculations of hadronic transport models with a baryonic mean-field qualitatively reproduced the data. These observations imply the vanishing of partonic collectivity and a new EoS, likely dominated by baryonic interactions in the high baryon density region.

Figure: v2 scaled by the number of constituent quarks, v2/nq, as a function of scaled transverse kinetic energy ((mT-m0)/nq) for pions, kaons, and protons from Au+Au collisions in 10-40% centrality at $\sqrt{s_{\rm NN}}$ = 3, 27, and 54.4 GeV for positively charged particles (left panel) and negatively charged particles (right panel.) The measurements are in the rapidity range |y| < 0.5 at 27 and 54.4 GeV, and in $-$0.5 < y < 0 at 3 GeV. Color dashed lines represent the scaling fit to data from Au+Au collisions at 7.7, 14.5, 27, 54.4, and 200 GeV from STAR experiment at RHIC. Statistical and systematic uncertainties are shown as bars and gray bands, respectively.

Posted March 10, 2022

STAR focus: Measurement of inclusive electrons from open heavy-flavor hadron decays in p+p collisions at $\sqrt{s}$ = 200 GeV with the STAR detector

Studying the properties of the Quark Gluon Plasma (QGP) created in heavy-ion collisions is a main goal of the RHIC physics program, and heavy flavor quarks are an excellent probe to the QGP as they are produced mainly at the early stage of the collision due to their large masses. Measurement of heavy quark production in p+p collisions serves as an important reference to similar measurements in heavy-ion collisions for understanding the nature of interactions between heavy quarks and the QGP. It also provides an important testing ground for perturbative Quantum Chromodynamics (pQCD) calculations. Electrons from semi-leptonic decays of heavy-flavor hadrons, referred to as heavy flavor electrons (HFEs), are measured as proxies for heavy quarks.

Recently, the STAR collaboration published "Measurement of inclusive electrons from open heavy-flavor hadron decays in p+p collisions at $\sqrt{s}$ = 200 GeV with the STAR detector" in Phys. Rev. D 105, 032007 (2022). In this publication, a new measurement of the production cross section for inclusive HFEs as a function of transverse momentum (pT) at midrapidity (|y| < 0.7) for 2.5 < pT < 10 GeV/c in p+p collisions at $\sqrt{s}$ = 200 GeV is reported. The result without subtracting the J/ψ, Υ, and Drell-Yan contributions is consistent with previously published STAR and PHENIX results, with significantly improved precision above 6 GeV/c. The result, with all background hadronic decay sources removed, is qualitatively consistent with the upper limit of fixed-order next-to-leading logarithm (FONLL) calculations, providing further constraints on theoretical calculations. Furthermore, this result provides a precise reference for nuclear modification factor measurements of HFEs in heavy-ion collisions. It also facilitates a study on the separation of the charm and bottom quark contributions to HFEs in p+p collisions.

Figure: (a) The HFE cross section at STAR in p+p collisions at $\sqrt{s}$ = 200 GeV from 2012 (filled circles) and the FONLL calculation (curves). (b) Ratio of data over FONLL calculation. The vertical bars and the boxes represent statistical and systematic uncertainties, respectively.

Posted March 8, 2022

STAR focus: Search for the chiral magnetic effect via charge-dependent azimuthal correlations relative to spectator and participant planes in Au+Au collisions at 200 GeV

The STAR Collaboration recently published “Search for the chiral magnetic effect via charge-dependent azimuthal correlations relative to spectator and participant planes in Au+Au collisions at 200 GeV” in Physics Review Letters. 128, 092301 (2022).

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum chromodynamics. Such a strong magnetic field may exist at early times in non-central relativistic heavy-ion collisions, on average perpendicular to the reaction plane. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy (elliptic flow). This flow background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. STAR reported such measurements in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV in the recently published Letter. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral collisions. Some indication of finite CME signals is seen in mid-central collisions. The results are shown in the figure below.

Figure: The flow-background removed <fCME >, the fraction of CME signal contained in the inclusive Δγ measurement, (a) and <ΔγCME > (b) signal in 50-80% (open markers) and 20-50% (solid markers) centrality Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, extracted by various analysis methods (FE: full event, SE: subevent) and kinematic cuts. Error bars show statistical uncertainties; the caps indicate the systematic uncertainties.

In 2009 STAR made the first measurement of the CME-sensitive charge separation observable, the Δγ correlator. Since then, many developments have been made to reduce or eliminate backgrounds in the Δγ measurements. These backgrounds are caused by particle cluster correlations, the effect of which is made reaction-plane dependent by the elliptic flow of those clusters. A new approach is exploited in the current Letter, measuring the Δγ correlators with respect to the first-order harmonic plane from the zero-degree calorimeters and the second-order harmonic plane from the Time Projection Chamber (TPC) in STAR. Because the former aligns better with the spectator plane and the latter aligns better with the participant plane, these measurements contain different amounts of the plane sensitive flow backgrounds and the magnetic field sensitive CME signal, and can thus be used to extract the possible CME. Full events, where the particles of interest and the second-order harmonic plane are both reconstructed from the entire TPC, and subevents, where they are reconstructed from two separate halves of the TPC, are analyzed. The subevents are less affected by nonflow correlations, those that are unrelated to the harmonic planes. As shown in the figure, the possible CME signal is smaller in subevents than in full events, indicating that nonflow effects may still be present in those measurements. A recent study of nonflow effects using models as well as experimental data, suggests that the remaining nonflow effects in the subevent CME measurements could even be negative. Further investigations are warranted to fully assess the significance of these measurements.

Posted March 8, 2022

STAR focus: Measurement of cold nuclear matter effects for inclusive J/ψ in p+Au collisions at √sNN = 200 GeV

The Quark-Gluon Plasma (QGP), a deconfined state of quarks and gluons, is believed to have existed momentarily after the Big Bang. Creating the QGP in ultra-relativistic heavy-ion collisions and understanding its properties is one of the main goals of the RHIC physics program. Among various probes used to study the QGP, quarkonia, bound states of heavy quark and anti-heavy quark pairs, play a unique role as they are expected to be dissociated in the QGP by the surrounding partons, leading to experimentally observable yield suppression. Interpretation of quarkonium measurements in heavy-ion collisions is complicated by other effects, which need to be quantified in order to infer QGP properties from these measurements. One such example is the so-called cold nuclear matter (CNM) effects, referring to the influence brought by the presence of the nucleus in the collision but not related to the QGP formation.

STAR recently published a new measurement of the CNM effects for inclusive J/Ψ mesons at midrapidity in p+Au collisions at the center of mass energy per nucleon-nucleon pair of 200 GeV (Phys. Lett. B 825 (2022) 136865). In p+Au collisions, the QGP is not expected to be created, and therefore measurement of the quarkoinum yield modification in these collisions can be used to quantify the CNM effects. J/Ψ mesons are reconstructed through the dimuon decay channel, taking advantage of the trigger capability provided by the Muon Telescope Detector in the RHIC 2015 run. The MTD dimuon trigger was able to sample the full luminosity delivered by RHIC for measuring J/Ψ down to zero transverse momentum (pT).

The J/Ψ yield suppression in p+Au collisions with respect to that in p+p collisions is shown as red stars in the figure. The magnitude of the suppression is about 30% below 2 GeV/c, which gradually goes away above 3 GeV/c, indicating that the CNM effects are minimal for J/Ψ mesons at high-pT. Comparison to a similar measurement from 0-20% central Au+Au collisions, shown as blue circles in the figure, reveals that the observed strong J/Ψ suppression above 3 GeV/c in heavy-ion collisions is mostly due to the hot medium effects, providing strong evidence for the formation of the quark-gluon plasma in these collisions.

Figure: Inclusive J/Ψ RpAu (filled stars) and RdAu (open circles) as a function of pT compared to the J/Ψ RAA (filled circles) measured in 0-20% central Au+Au collisions at 200 GeV. The error bars and open boxes around data points represent statistical and systematic uncertainties, while the filled boxes at unity display the global uncertainties for each dataset.

Posted January 20, 2022

STAR focus: Investigation of Experimental Observables in Search of the Chiral Magnetic Effect in Heavy-ion Collisions in the STAR experiment

The chiral magnetic effect (CME) is predicted to occur as a consequence of a local violation of $\cal P$ and $\cal CP$ symmetries in the strong interaction amidst a strong electromagnetic field generated in relativistic heavy-ion collisions, and may survive the expansion of the quark-gluon plasma fireball and be detected in experiments. Over the past two decades, the experimental searches for the CME have aroused extensive interest at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), and so far there is no conclusive evidence for its existence.

Multiple observables have been proposed to study the CME, and it is desirable to have a comprehensive understanding of their sensitivities and the connection between them. We have published a methodology study on the "Investigation of Experimental Observables in Search of the Chiral Magnetic Effect in Heavy-ion Collisions in the STAR experiment" (Chinese Phys. C 46 014101) In this publication, we have presented the relation between the three pertinent experimental approaches: the $\gamma$ correlator, the $R$ correlator, and the signed balance functions via analytical derivation. The equivalence in the core components among these methods have been verified by exploiting both simple Monte Carlo simulations and a realistic event generator Event-By-Event Anomalous-Viscous Fluid Dynamics (EBE-AVFD). This study indicates that, if all the three methods are implemented on equal footings, they should exhibit similar sensitivities.

Additionally, we have applied the STAR frozen codes to the EBE-AVFD events of isobar collisions to investigate these observables' sensitivities to the CME signal and the background contributions. The exactly same codes have been used in the blind-analysis procedure by STAR to analyze the isobar data. The figure below shows the sensitivities of the observables to the difference in the hypothetical CME strength between Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\rm NN}} = 200$ GeV from 30-40% central events generated by EBE-AVFD.

It is found that $\Delta\gamma_{112}$ and $r_{\rm lab}$ have similar significance levels in accordance with the CME signal. $\sigma_{R2}^{-1}$, as was implemented in the STAR frozen code, shows a relatively lower sensitivity to the signal increase, owing to the implementation details. This study provides a reference point to gauge the STAR isobar-collision results on the CME which was just published in January, 2022.

Figure: EBE-AVFD calculations of $\Delta \gamma_{112}$ (a), $\sigma^{-1}_{R2}$ vs $n_{5}/s$ (c) and $r_{\mathrm{lab}}$ (e) as functions of $n_{5}/s$ for 30-40% isobar collisions at $\sqrt{s_{\rm NN}} = 200$ GeV, together with their corresponding ratio between Ru+Ru and Zr+Zr in the bottom panels. $n_{5}/s$ is the input parameter for the CME strength in the EBE-AVFD model. In panel (b), (d) and (f), the $2^{\rm nd}$-order polynomial fit function illustrates the rising trend starting from (0, 1).

Posted January 18, 2022

STAR focus: A new PYTHIA 8 underlying event tune for RHIC energies

STAR collaboration members have published “PYTHIA 8 underlying event tune for RHIC energies” in Physical Review D 105, 016011 (2022).

In this publication, a new PYTHIA 8 tune, dubbed the “Detroit tune”, is produced that is able to describe underlying event data at mid-rapidity at center-of-mass energies reached at RHIC and also the Tevatron using new RIVET analyses for STAR measurements. This is an important achievement because accurately modeling the underlying event in proton-proton collisions is essential, and has not been achieved at these low collision energies in existing PYTHIA 8 tunes. The Detroit tune is also compared to existing tunes at LHC energies, and is either comparable or slightly better in reproducing underlying event data in events with high pT jets or hadrons.

Additionally, in this publication the validity of the Detroit and default Monash tunes are explored in data at forward rapidity. Compared to data from BRAHMS and STAR at forward rapidities, neither tune is able to reproduce the data. This discrepancy is important to recognize as with the STAR forward rapidity detectors beginning to take data, having a simulation that is able to accurately reproduce forward and mid-rapidity data will be essential for analyses. This opens the door for future tuning exercises, and also for MC authors and phenomenologists to search for an accurate representation of both the hard and soft particle production in proton-proton collisions.

In parallel to the publication, the authors have made all the RIVET analysis code utilized in the tuning procedure public on the STAR Github page. This will provide the necessary materials for anyone to further study event generator tuning with these RHIC data.

Figure: Top: Average charged track multiplicity as a function of leading jet pT in the azimuthal region transverse to the jet axis measured by STAR. The two curves show the PYTHIA 8 distributions with the new Detroit tune (red solid) and default Monash tune (blue dashed). Bottom: The ratio of the PYTHIA 8 curves with respect to data. The shaded yellow band shows the full data uncertainties.

Posted January 13, 2022

STAR focus: Measurement of the Sixth-order Cumulant of Net-proton Multiplicity Distributions in Au+Au Collisions at √sNN = 27, 54.4, and 200 GeV at RHIC

One of the ultimate goals of high energy physics is to understand the phase structure of Quantum Chromodynamics (QCD). The STAR collaboration's recent publication in Phys. Rev. Lett. 127, 262301 throws light on the nature of quark-hadron transition at low baryon chemical potential through the first measurement of the sixth to second-order cumulant (C6/C2) ratio of net-proton multiplicity distributions.

According to theoretical models and lattice QCD calculations, the ratio of the sixth to second-order susceptibility of baryon numbers becomes negative at near the crossover transition temperature. Experimentally, one can check the prediction by measuring the sign of C6/C2 of the net-proton distribution, a good proxy of net-baryon distributions.

The figure below shows the centrality dependence of net-proton C6/C2 in Au+Au collisions at the center-of-mass energies of 27, 54.4, and 200 GeV. The C6/C2 values are flat and consistent with zero within uncertainties for 27 and 54.4 GeV. On the other hand, the central values of C6/C2 for 200 GeV are progressively negative from peripheral to central collisions. This systematic negative sign trend of net-proton C6/C2 cannot be explained by the hadron transport model (UrQMD) and is qualitatively consistent with the expectations from lattice QCD calculations. Hence our experimental results could suggest a smooth crossover transition at the RHIC top energy, where the baryonic chemical potential is ~ 20 MeV.

Figure: Centrality dependence of net-proton C6/C2 in Au+Au collisions at 27, 54.4, and 200 GeV. The inset shows an expanded plot for 40-80% centrality. Results from UrQMD transport model are shown in a gray band. Corresponding baryon number fluctuations from lattice QCD calculations are shown in a blue band for T = 160 MeV with μB < 110 MeV

Posted December 20, 2021

STAR focus: Invariant Jet Mass Measurements in pp Collisions at √s = 200 GeV at RHIC

In a new paper published in Phys. Rev. D 104, 052007 (2021), the STAR Collaboration measured for the first time the invariant mass, M, of inclusive jets in pp collisions at √s = 200 GeV with data taken in 2012. The anti-kT algorithm from the FastJet package was used to cluster the collimated hadrons that result from parton showers into collective objects called jets. The invariant mass of a jet is defined as the magnitude of the four-momentum sum of all of the constituents falling within the jet radius, R. By comparing jet substructure measurements — with jet mass as one such substructure observable — with calculations or event generators, we can improve our understanding of both perturbative QCD (parton production, shower) and non-perturbative QCD (hadronization), and further tune our models to describe the data. To make this comparison possible the data had to be corrected for detector effects, in this case using a two-dimensional iterative Bayesian deconvolution method in the RooUnfold package, which brought the jet mass and transverse momentum, pT, to particle-level simultaneously. Additionally, STAR presented the fully corrected mass of SoftDrop "groomed" jets, which have soft radiation reduced, allowing for better comparison to results from perturbative calculations and event generators at parton-level.

Figure: Comparison between hadron-level fully corrected STAR data and a calculation at next-to-leading-log accuracy at parton-level as well as parton-level Monte Carlo, for jet mass, M, and SoftDrop-groomed jet mass, Mg, of anti-kT jets in pp collisions at √s = 200 GeV.

The jet mass results were presented for three selections of the jet radius to tune the amount of typically soft radiation unassociated with the initial partonic scattering contaminating the jet, and the relative importance of radiation leaving the jet, or exiting and then re-entering via a subsequent splitting. Three jet pT selections are shown as well, to study the dependence of the jet mass on its momentum. We find, as expected, that the jet mass increases both with increasing R and pT, due to the increased phase space for radiation. For each R and jet pT, the jet mass is compared to leading-order Monte Carlo event generators PYTHIA-6 (tuned to STAR), PYTHIA-8 (LHC tuned), and HERWIG-7 (LHC tuned). We find that PYTHIA-6 describes data accurately in almost all cases, while HERWIG-7 and PYTHIA-8 in general predict smaller and larger jet mass, respectively, than what is observed. This demonstrates the need for further tuning of these event generators at RHIC.

Finally, a comparison (see figure) was also made to parton-level, with both parton-level PYTHIA-8 and a calculation from Lee and Ringer. This comparison demonstrated the large impact of hadronization on the jet mass as well as the efficacy of SoftDrop grooming in suppressing the non-perturbative radiation in the jets to reduce the effect of hadronization. These new data can be used to extract a non-perturbative shape function for future calculations performed at RHIC energies.

Posted October 4, 2021

STAR focus: Measurement of Momentum and Angular Distributions of e+e- pairs from Linearly Polarized Photons

Only a handful of fundamental interactions between light and matter are allowed by the theory of quantum electrodynamics, all of which but one have been observed in the 80 or so years since their prediction. The Breit-Wheeler process, the simplest mechanism for converting 'light quanta' into matter and antimatter, has eluded observation for decades, despite being hotly pursued.The idea that you can create matter from smashing together light is a striking demonstration of the physics immortalized in Einstein's famous E=mc2 equation, which revealed that energy and matter are two sides of the same coin.

Recently, the STAR collaboration published "Measurement of Momentum and Angular Distributions of e+e- pairs from Linearly Polarized Photons" in Physical Review Letters, in which presents observation of the Breit-Wheeler process in heavy-ion collisions for the first time. This discovery was made possible by a unique analysis which measured the quantum spin-momentum correlations of the produced e+e- pair, revealing a striking 4th order angular modulation (See Figure). Virtual photons live only briefly as they mediate the electromagnetic force and carry a virtual mass. While virtual photons can be in the helicity 0 state, due to their virtual mass, real photons cannot, and instead must have +/-1 unit of helicity. This difference has a profound impact on the produced e+e- pair, since the quantized spin of the colliding photons becomes encoded in the final momentum of the produced electron and positron, resulting in the observed modulated emission angle ($\Delta\phi$). As the figure shows, the STAR data agree with calculations of the Breit-Wheeler process (QED), which predicts a strong cos$4\Delta\phi$ modulation from the collision of linearly polarized photons.

The measured cos$4\Delta\phi$ modulation proves another tantalizing prediction from decades ago. Heavy-ion collisions have long been expected to produce the strongest magnetic fields in the Universe, of order 1015 Tesla. Physicists in the 1930's predicted that photons shooting through such strong magnetic fields can be "bent", despite the photon itself not being charged - and therefore not directly interacting with the electromagnetic field. However, in quantum mechanics, a real photon can briefly fluctuate into an e+e- pair which can interact with the strong electromagnetic fields. The key prediction for this effect, called vacuum birefringence, is that the photon's path is split depending on the angle between the photon's polarization and the magnetic field direction. In the recent STAR measurement, the colliding photons result from the highly boosted electromagnetic fields of the heavy ions, so the photon's polarization direction is directly related to the classical electric and magnetic field direction. Therefore, the observed cos$4\Delta\phi$ modulation can be understood in terms of the absorption of light when the polarization of the photon (from one ion) is parallel vs. perpendicular to the magnetic field direction (produced by the other ion). This absorption effect is directly related to vacuum birefringence, and provides the first experimental verification that heavy-ion collisions really do produce ultra strong magnetic fields (approximately 1015 Tesla).

Read more about this discovery:
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Posted September 10, 2021

STAR focus: Observation of Ds/D0 in Au+Au collisions at √sNN = 200 GeV

The STAR Collaboration has recently published an article titled "Observation of Ds/D0 in Au+Au collisions at √sNN = 200 GeV” in Phys. Rev. Lett. 127, 092301.

Charm quarks are produced on timescales shorter than the Quark-Gluon Plasma (QGP) formation in heavy-ion collisions and they subsequently experience the whole evolution of the QGP matter, making them an excellent probe to study the transport properties of the QGP. In the QGP medium, one expects a different hadronization mechanism from p+p collisions through the recombination of charm quarks and light/strange quarks (namely coalescence hadronization) to dominate at low pT (< 5 GeV/c) and fragmentation hadronization to dominate at higher pT. Due to the enhanced strange-quark abundance in the QGP, an increased Ds production in heavy-ion collisions relative to p+p collisions has been predicted in case of hadronization via quark recombination. Comparing the Ds/D0 yield ratio in heavy-ion collisions with that in p+p therefore helps us understand the QGP effects on charm-quark hadronization.

In this paper we present the first measurement of Ds production and Ds/D0 yield ratio as a function of pT for different collision centralities at midrapidity (|y| < 1) in Au+Au collisions at √sNN = 200 GeV. A clear enhancement of the Ds/D0 yield ratio is found compared to PYTHIA simulations of p+p events at the same collision energy. For the Ds/D0 ratios integrated over 1.5 < pT < 5 GeV/c in the 10%–60% centrality range, the significance of this observation is more than 5 standard deviations. The pT-integrated Ds/D0 ratio is compatible with predictions from a statistical hadronization model. The enhancement, and its pT dependence, can be qualitatively described by model calculations incorporating thermal abundance of strange quarks in the QGP and coalescence hadronization that includes charm quarks. These results suggest that recombination of charm quarks with strange quarks in the QGP plays an important role in Ds-meson production in heavy-ion collisions.

Figure: (a) Ds/D0 yield ratio as a function of pT compared to various model calculations from He/Rapp (0%–20%), Tsinghua, Catania, and Cao-Ko in 0%–10% centrality interval of Au+Au collisions, and PYTHIA prediction in p+p collisions at √sNN = 200 GeV. (b) Ds/D0 yield ratio as a function of pT compared to model calculations from Tsinghua in 20%–40% (solid circles) and 40%–80% (open circles) centrality intervals of Au+Au collisions at √sNN = 200 GeV. Vertical bars and brackets on data points represent statistical and systematic uncertainties, respectively.

Posted August 30, 2021

STAR focus: Azimuthal Anisotropy Measurements of Strange and Multi-strange Hadrons at Midrapidity in Collisions of U+U Nuclei at √sNN = 193 GeV

The STAR Collaboration has recently published the results on azimuthal anisotropy measurements of strange and multi-strange hadrons at midrapidity (|y| < 1.0) in collisions of U+U nuclei at √sNN = 193 GeV in Physical Review C 103, 064907 (2021).

Azimuthal anisotropy measurements of identified hadrons are excellent probes to understand the quark-gluon plasma (QGP) medium and help in constraining transport and hydrodynamic model simulations. Multi-strange hadrons containing only constituent strange quarks, especially the φ-meson($s\bar{s}$) and Ω($sss$), provide a clean probe to investigate partonic collectivity of the QGP phase in relativistic heavy-ion collisions. One can understand the initial conditions in heavy-ion collisions via varying the collision system size. This could be achieved by colliding uranium nuclei which possess a prolate shape, consequently yielding collision configurations (body-body collisions) in which the initial overlap region is not spherical even in central collisions. Studying various collision shapes will provide an additional constraint for the initial conditions in models.

Figure: Flow coefficients v2, v3, and v4 as a function of transverse kinetic energy KET/nq for various particles at mid-rapidity (|y| < 1) in U+U collisions at √sNN = 193 GeV, scaled by the number of constituent quarks (nq) to the power n/2. Left panels represent results for minimum bias (0-80%) and right panels for centrality class (10-40%).

We reported systematic measurements of the transverse momentum (pT) dependence of flow coefficients (v2, v3, and v4) in U+U collisions for minimum bias and three different centrality intervals, compared with Au+Au collisions, hydrodynamic, and transport model calculations. We present the number of constituent quark scaling (NCQ) of vn coefficients for strange and multi-strange hadrons in U+U collisions at √sNN = 193 GeV. Our measurements show that the NCQ scaling holds within experimental uncertainties for each harmonic order n = 2, 3 and 4. The vn/nqn/2 vs. KET/nq values lie on a single curve for all the particle species within ±15%. The measured NCQ scaling of vn coefficients indicates the development of partonic collectivity during the QGP phase in heavy-ion collisions. This observed scaling also suggests the formation of hadrons through quark coalescence in the intermediate pT range (2.0 < pT (GeV/c) < 4.0). Although there are considerable differences in the collision geometry between U+U and Au+Au, the hydrodynamical evolution and the coalescence mechanism for hadron formation remain key features of QGP drops formed in nucleus-nucleus collisions.

Posted June 19, 2021

STAR focus: Transverse Single-Spin Asymmetries of π0 and Electromagnetic Jets at Forward Rapidity

The STAR Collaboration has recently published an article titled "Measurement of transverse single-spin asymmetries of π0 and electromagnetic jets at forward rapidity in 200 and 500 GeV transversely polarized proton-proton collisions” in Physical Review D 103 092009 (2021).

Large transverse single-spin asymmetries (TSSA) had been observed for hadrons produced in transversely polarized hadron-hadron collisions. Based on the QCD framework, the TSSAs can originate from two possible sources. One being an initial state effect which is correlated to the parton distribution functions and another being a final state effect related to the fragmentation process. The classical measurement of a π0 TSSA usually mixes the two effects. This paper reports measurements of the π0 TSSA together with the TSSA for EM-jets and a TSSA sensitive to the Collins mechanism. The latter two results are sensitive to either the initial or final state effect respectively. The π0 TSSA result shows a weak energy dependence. The jet TSSA is small but non-zero, and the Collins asymmetry is consistent with zero.

In this paper, we also present a novel phenomenon of the π0 TSSA. In the analysis, it was found that the π0 TSSA is not only governed by its kinematics (energy and transverse momentum) but also by the event topology. The π0s which have no other energy around them are defined as “isolated π0s”, which tend to have a larger TSSA than “non-isolated π0s”. The figure below shows that both for √s = 200 GeV and 500 GeV, the π0 TSSAs for the isolated π0s are significantly larger than for non-isolated π0s. This difference suggests different underlying mechanisms for the two types of π0, which challenges our understanding to the origin of the π0 TSSA. Diffractive production may be a suspect for the source of isolated π0, this result certainly needs further efforts from both theorists and experimentalists.

Figure: The transverse single-spin asymmetry as a function of xF for the isolated and non-isolated π0 in transversely polarized proton-proton collisions at √s = 200 and 500 GeV. The error bars are statistical uncertainties only. A systematic uncertainty up to 5.8% of AN for each point is smaller than the size of the markers. Theory curves based on a recent global fit are also shown. The average pT of the π0 for each xF bin is shown in the lower panel. The isolated π0 TSSAs are significantly larger than those for non-isolated π0s.

Posted June 3, 2021

STAR focus: Longitudinal Double-Spin Asymmetry for Inclusive Jet and Dijet Production in Polarized Proton Collisions at √s = 200 GeV

How is the spin of the proton distributed among its quark, anti-quark, and gluon constituents? The STAR experiment addressed this fundamental question using collisions of high-energy polarized protons.

We recently published results on the “Longitudinal Double-Spin Asymmetry for Inclusive Jet and Dijet Production in Polarized Proton Collisions at √s = 200 GeV” in Phys. Rev. D 103, L091103 (2021) highlighted as PRD Editors' Suggestion. The data give insight in the gluon spin contribution to the proton spin.

We measured the asymmetries in the differential production cross sections of inclusive jet and dijet probes for different longitudinal spin configurations of the colliding proton beams as a function of jet transverse momentum and dijet invariant mass. Since gluon-gluon and gluon-quark scattering contributions dominate the production of these probes in the STAR environment, the data provide sensitivity to the gluon spin contribution to the proton spin.

Left Figure: Results of the double-spin asymmetry for dijets as a function of dijet invariant mass for two different event topologies. The topologies probe complementary gluon fractional momenta. The green square markers show the new results from data collected in 2015. The blue triangle markers show our prior results based on data collected in 2009 and are seen to be in good agreement. The curves show dijet asymmetry expectations from the DSSV14 and NNPDFpol1.1 theory collaborations and the band indicates the size of the uncertainty in the NNPDFpol1.1 expectation.

Our prior results provided first evidence for a positive polarization of the gluons in the polarized nucleon for gluon fractional momenta larger than 0.05 upon their inclusion in global analyses. Our new results have an approximately twice larger figure of merit, with improved systematic uncertainties, and thus considerably strengthen this evidence. Since we concluded our data taking with longitudinally polarized protons in 2015, these data are anticipated to provide the most precise insights in gluon polarization well into the future, likely until the future Electron-Ion Collider comes online.

Posted May 27, 2021

STAR focus: Methods for a blind analysis of isobar data collected by the STAR collaboration

The STAR Collaboration has recently published “Methods for a blind analysis of isobar data collected by the STAR collaboration” in Nuclear Science and Techniques.

For more than a decade, STAR has searched for evidence of chiral magnetic effects (CME), which refer to induction of an electric current by the magnetic field in a chiral system. In 2018 STAR collected data from isobaric nuclei, ${^{96}_{44}Ru}+{^{96}_{44}Ru}$ and ${^{96}_{40}Zr}+{^{96}_{40}Zr}$. Varying the number of protons while maintaining a consistent number of nucleons presents an opportunity to vary the initial magnetic field while keeping background contributions approximately the same.

For the first time, STAR has implemented blind analyses of these data for CME-related studies. Many of the typical blind analysis methods are not well-suited to the specific needs of the isobar CME analyses. For example, many blind analysis methods “hide” or “offset” variables or information needed to gain sensitivity to signals. For the isobar analysis, randomizing the sign of charged-particle tracks or randomizing particle azimuthal angle would blind information needed for the CME signal. However, so doing would also prevent charge-dependent efficiency corrections and destroy correlations from secondary decays, respectively. Consequently, STAR developed a new method for blind analysis of isobar collision data.

The STAR isobar blind analysis method is a three-step process. In the first step, analysts are provided output files composed of events from a mix of the two isobar species. To the extent possible, the order of events respects temporal changes in running conditions. Events are randomly rejected at the level of ~10% to prevent determining the species by simply counting the number of events associated with a particular run or event trigger.

Analysts use this mixed data sample to tune analysis code and time-dependent Q/A. In the second step, analysts are provided an “unmixed-blind” sample of data comprised of files that obscure the true run number—hence, the isobar species—but do not mix events across different runs. This sample enables species-blind run-by-run Q/A and only run-by-run corrections and code alteration directly resulting from these corrections are allowed at this stage. Once Q/A is complete and analysis of the run-by-run Q/A data are final, full un-blinding proceeds. In this third stage, physics results are produced with the previously tuned, vetted, and fixed analysis codes.

Work toward the blind analysis began, even before the isobar data were collected. To the extent possible, information pertaining to the isobar species was restricted during the run. In assembling the computing machinery for data production, information such as the data-taking run, RHIC fill, event timestamp, collision species, ZDC hit rates, etc., were obfuscated to blind the collision species. Physics analysts participated in a “mock-data challenge.” In this study, analysts were presented data from collisions at GeV, also collected in 2018, produced in the same three-step manner as intended for the blind isobar productions. This exercise served as an opportunity for the software and computing team to develop, tune, and test the machinery necessary for the blind isobar data samples. Furthermore, the mock-data challenge served as an opportunity for analysts to test the feasibility of the methods to enable a robust data analysis for their particular physics observables. An example of a quality-assurance plot from the mock data challenge is shown, above.

The procedure described in our paper was accepted by the STAR institutional council in January 2018, prior to the start of the isobar collision runs. The isobar analysis is well underway following the procedure outlined in the published manuscript.

Posted May 17, 2021

STAR focus: Flow and interferometry results from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 4.5 GeV

The STAR Collaboration has recently published “Flow and interferometry results from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 4.5 GeV” in Physical Review C 103, 034908.

A large 1-kg block of VERY cold (-100℃=-148℉) ice is placed on a kitchen stove. It takes an hour for the temperature to change by 300℃. But 40 of those minutes are spent at a single temperature, in the phase transition from liquid to gaseous H2O.

If you heat water at a constant rate, the temperature rises at a nearly constant rate, until it reaches 100 degrees Celsius, at which point the temperature remains constant for some time, while the water vaporizes. During this process, all of the energy goes into the so-called Latent Heat of Vaporization, rather than increasing the temperature. After the phase of the system has changed from liquid to gas, the temperature again increases at a nearly constant rate. The same process, in reverse, occurs when the system cools as energy leaves the system—the temperature drops in one phase, then remains constant for some time, then continues to drop. Latent Heat is a feature of all First Order Phase Transitions (1oPT), and its magnitude is driven by the underlying physics of the system—in this case, the interactions between H2O molecules. Scientists often focus intensely on phase transitions to understand the fundamental nature of a system.

It is currently believed that the transition from the Quark Gluon Plasma (QGP) phase to the hadronic phase is a 1oPT with a Latent Heat driven by the interaction between quarks and gluons. Identifying and quantifying this transition has been one of the driving motivations behind experiments at RHIC. Since at least 30 years ago, theoretical modeling has suggested that the system created in the collision will have a “long” lifetime if the system is near the transition temperature. However the QGP only lives for about a billionth of a trillionth of a second—about the time required for light to cross an atomic nucleus! How can we measure those timescales? Nuclear physicists use quantum correlations between subatomic particles, called pions, emitted from the QGP to measure spatial and temporal scales, a technique known as “femtoscopy”. By systematically changing the initial temperature of the QGP—at RHIC, the beam energy is systematically varied—we look for the difference between the “out” and the “side” radius to first rise, and then fall.

The difference between the "out" and "side" radii is a measure of the lifetime of QGP. It is seen to grow, and then fall, as the collision energy is increased, in agreement with theoretical expectations for a first-order phase transition.

In a recent publication, the STAR Collaboration reports a compelling observation of this long-sought signal that supports the phase transition picture. This observation was possible due to several key features of the experiment. The unprecedented flexibility of the RHIC facility allowed high-statistics, precision measurements over a wide range of closely spaced energies. This included running the experiment in a novel configuration, in which only one nuclear beam impinges on a stationary gold foil. This so-called “fixed target” measurement is the focus of the recent STAR paper. The result at the low collision energy accessible in this mode was crucial, as it defines the left side of the peak structure with much smaller uncertainty than was possible in earlier measurements at similar energies. These earlier results are shown in the figure as open points; their error bars are clearly too large to allow detection of the underlying peak.

After years of systematic experimentation, we now have experimental observation and measurement of the timescale signature consistent with a first-order phase transition in heavy ion collisions. While systematic theoretical study is needed to understand the signal and its physical implications, STAR’s recent publication represents a big step towards mapping the phase structure of hot QCD matter.

Posted May 8, 2021

STAR focus: Tantalizing Signs of Phase-change 'Turbulence' in RHIC Collisions

Sample of BNL news article by Karen McNulty Walsh and Peter Genzer. Read the full article here

UPTON, NY—Physicists studying collisions of gold ions at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science user facility for nuclear physics research at DOE’s Brookhaven National Laboratory, are embarking on a journey through the phases of nuclear matter—the stuff that makes up the nuclei of all the visible matter in our universe. A new analysis of collisions conducted at different energies shows tantalizing signs of a critical point—a change in the way that quarks and gluons, the building blocks of protons and neutrons, transform from one phase to another.

The STAR detector at the U.S. Department of Energy's Brookhaven National Laboratory

The findings, just published by RHIC’s STAR Collaboration in the journal Physical Review Letters, will help physicists map out details of these nuclear phase changes to better understand the evolution of the universe and the conditions in the cores of neutron stars.

“If we are able to discover this critical point, then our map of nuclear phases—the nuclear phase diagram—may find a place in the textbooks, alongside that of water,” said Bedanga Mohanty of India’s National Institute of Science and Research, one of hundreds of physicists collaborating on research at RHIC using the sophisticated STAR detector.

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Posted Mar 9, 2021

STAR focus: Measurements of $W$ and $Z/\gamma^*$ cross sections and their ratios in $p+p$ collisions at RHIC

The STAR Collaboration has recently published “Measurements of $W$ and $Z/\gamma^*$ cross sections and their ratios in $p+p$ collisions at RHIC” in Phys. Rev. D 103, 012001.

One of the fundamental goals of nuclear physics is to understand the proton’s structure and dynamics. Parton distribution functions (PDFs) of the proton account for the probability of finding a parton at a given fraction of the proton’s momentum, $x$, and four-momentum transfer, $Q^2$ . Although PDFs have become more precise, there are still kinematic regions where more data are needed to help constrain global PDF extractions, such as the ratio of the sea quark distributions $\bar{d}/\bar{u}$ near the valence region. Furthermore, different measurements appear to suggest different high-$x$ behaviors of this ratio. The $W$ boson cross-section ratio ($W^+/W^-$) is sensitive to the $\bar{d}/\bar{u}$ distributions at large $Q^2$ . Such a measurement can be used to help constrain the $\bar{d}/\bar{u}$ ratio.

Through $W$ and $Z$ boson production in $p+p$ collisions at a center-of-mass energy of 510 GeV, STAR has measured $W$ and $Z$ cross sections via the boson's leptonic decay channel from the 2011, 2012, and 2013 RHIC data sets. The combined result for the $W$ cross-section ratio is shown in Fig. 1, along with comparisons to several PDF predictions. A PDF reweighting study, using the new $W^+/W^-$ measurement, was done to provide an initial assessment of the data's sensitivity for $\bar{d}$, $\bar{u}$, $\bar{u}-\bar{d}$, and $\bar{d}/\bar{u}$ PDF distributions. The reweighting study shows modest constraining power on the PDFs. However, a proper assessment of the data's impact on PDF distributions requires a full global PDF analysis, including this STAR data in the fits used to extract the PDFs.

Fig. 1: STAR $W^+/W^-$ cross-section ratio measurements as a function of decay lepton pseudorapidity. Curves show various PDF predictions.

In addition to the $W$ cross-section ratio, STAR also reports on the measured $W/Z$ cross-section ratio, differential, and total $W$ and $Z$ cross sections, which are also sensitive to the proton's quark and antiquark distributions and can constrain proton PDFs further when used in a global PDF analysis.

Posted January 8, 2021

STAR focus: Measurement of Inclusive Charged-Particle Jet Production in Au + Au collisions at $\sqrt{s_{\rm{NN}}}=200$ GeV
The STAR Collaboration has recently published "Measurement of inclusive charged-particle jet production in Au + Au collisions at $\sqrt{s_{\rm{NN}}}=200$ GeV” in Physical Review C 102, 054913 (2020)

Collisions of heavy atomic nuclei at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN generate tiny droplets of matter under conditions of extreme temperature and density, similar to those of the early universe a few microseconds after the Big Bang, called the Quark-Gluon Plasma (QGP). The QGP, which has been studied at colliders for two decades, is a “perfect liquid,” with exotic properties. Among the most important experimental tools to study the QGP are jets, from rare hard scatterings of quarks and gluons from the colliding nuclei, and which are seen in the detectors as correlated sprays of particles. Jets generated in head-on (“central”) nuclear collisions plough through the QGP and interact with it before flying off to the detectors. This interaction causes the jets to lose energy (“jet quenching”), suppressing their production rate relative to that in proton-proton collisions and other simple systems, where a QGP is not expected to be formed.

Figure 1. STAR event display of a central (head-on) Au + Au collision with back-to-back jets.

Since the beginning of the RHIC program STAR has played a key role in the discovery and elucidation of jet quenching, and it continues to pioneer in this area. Figure 1 shows a STAR event display of a central Au + Au collision, including a pair of energetic jets that are back-to-back in azimuth at 90 degrees to the beam direction, as expected from the hard scattering of incoming quarks or gluons. While such jets are easy to see when highlighted in color, finding and measuring them accurately in the complex environment of Au + Au collisions is very challenging. Solving this problem has required the development of novel approaches to background suppression.

Using these novel techniques, STAR recently reported the first measurement of jet yield suppression in central Au + Au collisions at RHIC, opening up a new chapter in the study of jet quenching. Figure 2 shows the strong yield suppression of jets in central Au + Au collisions compared to that in glancing (“peripheral”) collisions (filled blue points). The figure also shows a similar measurement by ALICE for jets at the LHC (filled red) and for single charged particles at both RHIC and LHC (faded blue and red); such comparisons provide crucial constraints on theoretical models. These new data are a significant step towards meeting the goal of the 2015 NSAC Long Range Plan to explore the inner workings of the QGP using jet probes.

Figure 2. New STAR measurement of the yield suppression of jets in head-on Au + Au collisions (filled blue points). Absence of suppression corresponds to a value of unity. Also shown are similar measurements for jets at the LHC and single charged particles at both RHIC and LHC.

Posted December 1, 2020

STAR focus: Measurement of Groomed Jet Substructure Observables in pp Collisions at $\sqrt{s} = 200$ GeV with STAR

The STAR collaboration has recently published the first "Measurements of Groomed Jet Substructure Observables in pp Collisions at $\sqrt{s} = 200$ GeV with STAR" in Phys. Lett. B Volume 811.

This paper presents differential measurements of jets substructure via the SoftDrop momentum fraction ($z_{\rm{g}}$) and groomed jet radius ($R_{\rm{g}}$) for jets in the kinematic range $15 < p_{\rm{T}} < 60$ GeV/$c$ and for a variety of jet resolution parameters from $R=0.2$ to $R=0.6$. These substructure measurements are expected to be sensitive to the modeling of jet evolution in vacuum, including both perturbative and non-perturbative parts of the jet shower and serve as a baseline for future measurements in heavy ion collisions.

The measurements are fully unfolded and corrected to particle level in 2-dimensions i.e., $p_{\rm{T, jet}}$ and $z_{\rm{g}}$ or $R_{\rm{g}}$ via bayesian unfolding as implemented in the RooUnfold package. We find the STAR tuned PYTHIA 6 model is able to quantitatively reproduce the trends of both substructure observables in data whilst LHC tuned PYTHIA 8 and HERWIG 7 are unable to describe both measurements and end up predicting larger opening angle for jets (PYTHIA 8) or more symmetric splittings (HERWIG 7), respectively. These comparisons highlight the need for further tuning of MC models at varied center of mass energies and for understanding hadronization effects on jet evolution at RHIC kinematics.

Figure: Radial scans of the SoftDrop $z_{\rm{g}}$ in pp collisions at $\sqrt{s} = 200$ GeV for anti-k$_{\rm{T}}~R=0.2$ (left), $R=0.4$ (middle) and $R=0.6$ (right) jets of varying transverse momenta ($15 < p_{\rm{T, jet}}< 20$ GeV/$c$ and $30 < p_{\rm{T, jet}} < 40$ GeV/$c$ in the top and bottom rows respectively). The measurements are compared to various MC models shown in the colored lines.

The differential measurements enable radial and $p_{\rm{T}}$ scans of the jet substructure which show significant modifications to the $z_{\rm{g}}$ shape for jets with smaller resolution parameters and lower $p_{\rm{T, jet}}$ with respect to the ideal DGLAP splitting function, and do not reproduce the characteristic $1/z$ shape seen at higher $p_{\rm{T, jet}}$. We understand this as a consequence of significantly constricting the phase space for radiation within the reconstructed jets.

We also compared our measurements to recent calculations at next-to-leading-log accuracy for $R_{\rm{g}}$. These predictions are for jets at the parton level without non-perturbative corrections, with large systematic uncertainties arising from scale variations close to $\Lambda_{QCD}$. We see large discrepancies between the calculations and data for all of the jet resolution parameters and momenta except at the largest resolution parameter and highest $p_{\rm{T, jet}}$ where the scales are strictly perturbative. These comparisons highlight the need for more realistic calculations, including corrections arising from non-perturbative effects and higher-order corrections at small jet scales to further quantitatively understand the jet substructure at RHIC energies.

Posted December 1, 2020

STAR focus: Measurement of Inclusive $J/\psi$ Polarization in $p+p$ collisions at $\sqrt{s}$ = 200 GeV

The STAR Collaboration has recently published “Measurement of inclusive $J/\psi$ polarization in $p+p$ collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment” in Physical Review D 102, 092009.

The $J/\psi$ meson, a bound state of charm quark and its anti-quark, is one of the simplest systems in Quantum Chromodynamics (QCD). It was discovered in 1974 but its production mechanism in elementary particle collisions is still not fully understood. One of the difficulties is that the transition from charm and anti-charm quark interstate to the final-state color neutral meson involves soft processes, which cannot be calculated perturbatively and has to rely on modeling. The most popular models on the market are the Color Singlet Model (CSM), Color Evaporation Model and Non-relativistic QCD (NRQCD) framework. These models can describe the production yields measured from SPS to LHC energies reasonably well, but could not match the measured polarization consistently. Measurements of $J/\psi$ polarization provide powerful tests and constraints on modelling the $J/\psi$ production mechanism in vacuum.

The polarization parameters are measured via the angular distributions of the decayed leptons in the rest frame of the $J/\psi$ with respect to a certain quantization axis (reference frame). This paper presents the first measurement of inclusive $J/\psi$ polarization parameters $\lambda_\theta$, $\lambda_\phi$, $\lambda_{\theta\phi}$ in two reference frames (Helicity frame and Collis-Soper frame) via both di-electron and di-muon decay channels. The results are shown as a function of transverse momentum in the figure and compared to calculations from various theoretical models. The inclusive $J/\psi$’s do not exhibit significant transverse or longitudinal polarization. Among several model calculations, the NRQCD coupled with Color-Glass-Condensate (CGC) implementation agrees the best overall with data. The data presented in this paper provide additional tests and valuable guidance for theoretical efforts towards a complete understanding of the $J/\psi$ production mechanism in vacuum.

Figure: $J/\psi$ polarization parameters $\lambda_\theta$, $\lambda_\phi$, $\lambda_{\theta\phi}$ as a function of transverse momentum in Helicity frame (Left) and Collis-Soper frame (Right) in $p+p$ collisions at $\sqrt{s}$ = 200 GeV measured through di-electron and di-muon decay channels. Results are compared to various theoretical model calculations.

Posted November 30, 2020

STAR focus: Strange Hadron Production in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV

The STAR Collaboration has recently published "Strange hadron production in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV" in Physical Review C 102, 034909 (2020) and it is also highlighted as PRC Editors' Suggestion.

Strange hadrons are an excellent probe for identifying the phase boundary and onset of deconfinement in the QCD phase diagram. The STAR Collaboration has performed precision measurements of strange hadron ($\mathrm{K}^{0}_{\mathrm S}$, $\Lambda$, $\overline{\Lambda}$, $\Xi^-$, $\overline{\Xi}^+$, $\Omega^-$, $\overline{\Omega}^+$, and $\phi$) production at mid-rapidity ($|y| < 0.5$) in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV from the Beam Energy Scan Program at the Relativistic Heavy Ion Collider (RHIC). Transverse momentum spectra, averaged transverse mass, and the overall integrated yields of these strange hadrons have been extracted with high precision for all centralities and collision energies. Generally, the STAR BES data follow the trend of the previous measurements from AGS, SPS and RHIC. But the precision data also reveal new features, such as the deviation of $\overline{\Lambda}$ and $\Lambda$ $\left< m_{\rm T}\right>-m_0$ at lower energies and the non-monotonic energy dependence of $\Lambda$ and $\Xi^-$ $dN/dy$. The thermal model has been tested with the measured antibaryon-to-baryon ratios ($\overline{\Lambda}$/$\Lambda$, $\overline{\Xi}^+$/$\Xi^-$, $\overline{\Omega}^+$/$\Omega^-$), and then the temperature normalized strangeness and baryon chemical potentials at hadronic freeze-out ($\mu_{B}/T_{\rm ch}$ and $\mu_{S}/T_{\rm ch}$) are extracted for central collisions. The strange baryon-to-pion ratios are found to be consistent with the calculations of the statistical hadron gas model, and for $\Lambda$/$\pi$ ratio, consistent with hadronic transport models as well. The nuclear modification factors ($R_{\tiny{\textrm{CP}}}$) and antibaryon-to-meson ratios as a function of transverse momentum are presented for all collision energies, and they are shown in the left and right figures, respectively. The $\mathrm{K}^{0}_{\mathrm S}$ $R_{\tiny{\textrm{CP}}}$ shows no suppression for $p_{\rm T}$ up to 3.5 $\mbox{$\mathrm{GeV} / c$}$ at energies of 7.7 and 11.5 GeV. The $\overline{\Lambda}$/$\mathrm{K}^{0}_{\mathrm S}$ ratio also shows baryon-to-meson enhancement at intermediate $p_{\rm T}$ ($\approx$2.5 $\mbox{$\mathrm{GeV} / c$}$) in central collisions at energies above 19.6 GeV. Both observations suggest that there is likely a change of the underlying strange quark dynamics at collision energies below 19.6 GeV.

Left: The nuclear modifcation factor ($R_{\tiny{\textrm{CP}}}$) of strange hadrons at mid-rapidity ($|y|<0.5$) in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = $7.7-39$ GeV. Right: $\overline{\Lambda}$/$\mathrm{K}^{0}_{\mathrm S}$ ratio versus $p_{\rm T}$ at mid-rapidity ($|y|<0.5$) in different centralities from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = $7.7-39$ GeV.

The results in this paper significantly improve the experimental knowledge in the energy range where key features of the QCD phase diagram are nowadays being studied. For more details on the analysis and discussion of the results see the full paper here.

Posted October 5, 2020

STAR focus: Precise Measurement of the Mass Difference and the Binding Energy of the Hypertriton and Antihypertriton at STAR

In March 2020, the STAR Collaboration published "Measurement of the mass difference and the binding energy of the hypertriton and antihypertriton" in Nature Physics.

In this paper, we present two measurements from gold-gold collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 200$ GeV: the relative mass difference between $\rm^3_\Lambda H$ (the hypertriton) and $\rm^3_{\bar{\Lambda}}\overline{H}$ (the antihypertriton) (see Fig. 1), as well as the $\Lambda$ hyperon binding energy for $\rm^3_\Lambda H$ and $\rm^3_{\bar{\Lambda}}\overline{H}$ (see Fig. 2). The hypernucleus $\rm^3_{\Lambda}H$ is reconstructed through its mesonic decay channels $\rm^3_{\Lambda}H \rightarrow {^3}He + \pi^-$ (2-body decay) and $\rm^3_{\Lambda}H \rightarrow$ $ d + p + \pi^-$ (3-body decay). The significance $S/ \sqrt{S+B}$, where $S$ is signal counts and $B$ is background counts in the invariant mass window $2.986 - 2.996$ GeV$/c^{2}$, is 11.4 for $^3_\Lambda$H and 6.4 for $\rm^3_{\bar{\Lambda}}\overline{H}$. The signal counts from 2-body/3-body decay channels are about 121/35 for $^3_\Lambda$H and 36/21 for $\rm^3_{\bar{\Lambda}}\overline{H}$, respectively.

According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. A comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness for the first time, and we observe no deviation from the expected exact symmetry with precision of 10$^{-4}$.

This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron, and a $\Lambda$ hyperon. We measure the $\Lambda$ hyperon binding energy $B_{\Lambda}$ for the hypertriton, and find that it differs from the widely used value and from predictions, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon-nucleon interaction, and have implications for understanding neutron star interiors, where strange matter may be present.

Fig 1: Measurements of the relative mass-to-charge ratio differences between nuclei and antinuclei. The current measurement of the relative mass difference ${\Delta m}/m$ between $^3_\Lambda$H and $\rm^3_{\bar{\Lambda}}\overline{H}$ constrained by the existing experimental limits for decay daughters is shown by the red star marker. The green point is the new $^{3}$He result after applying the constraint provided by the present $^{3}_{\Lambda}$H result. The differences between $d$ and $\bar{d}$ and between $^3$He and $\rm^3\overline{He}$ measured by the ALICE collaboration are also shown. The two $^3$He - $\rm^3\overline{He}$ points are staggered vertically for visibility. The dotted vertical line at zero is the expectation from CPT invariance. The horizontal error bars represent the sum in quadrature of statistical and systematic uncertainties.

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Posted March 10, 2020

STAR focus: Polarization of Λ (Λ) hyperons along the beam direction in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV

The STAR Collaboration has recently published “Polarization of Λ (Λ) hyperons along the beam direction in Au+Au collisions at collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV” in Physical Review Letter 123, 132301.

This paper reports on the first ever measurements of the Λ hyperon polarization along the beam direction in heavy-ion collisions. In a non-central heavy-ion collision, the system expands stronger in the reaction plane direction compared to that in the out-of-plane direction, a phenomenon known as elliptic flow. Such nontrivial velocity fields lead to a non-zero vorticity component along the beam direction dependent on the azimuthal angle of fluid elements relative to the reaction plane (see left figure), and therefore to particle spin polarization. Spin polarization can be experimentally measured via hyperons parity-violating weak decay.

Left: A sketch illustrating the system created in a non-central heavy-ion collision viewed in the transverse plane (x-y), showing stronger in-plane expansion (solid arrows) and expected vorticities (open arrows). In this figure the colliding beams are oriented along the z-axis and the x-z plane defines the reaction plane. Right: The second Fourier sine coefficient of the polarization of Λ and Λ along the beam direction as a function of centrality in Au+Au collisions at 200 GeV.

Results using Λ hyperons at 200 GeV exhibit Λ's emission angle dependence of the polarization along the beam direction, as expected from elliptic flow, indicating a quadrupole pattern of the vorticity z-component. Right figure presents a sine modulation of the polarization relative to the reaction plane angle as a function of collision centrality. The observed phase of the sine modulation is opposite to some theoretical predictions, e.g. a multi-phase transport model (AMPT) and viscous hydrodynamic model. In contrast, the blast-wave model calculations representing the kinematic vorticity reproduce the modulation phase better. These results together with the results of the global polarization (Nature 548, 62 (2017), PRC98, 014910 (2018)) may provide information on the relaxation time needed to convert the vorticity to particle polarization.

Posted October 24, 2019

STAR focus: Measurements of inclusive $J/\psi$ suppression in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV through the dimuon channel

The STAR Collaboration has recently published "Measurements of inclusive $J/\psi$ suppression in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV through the dimuon channel at STAR" in Phys. Lett. B 797 (2019) 134917.

This publication reports the first measurement of inclusive $J/\psi$ suppression at mid-rapidity through the dimuon decay channel in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV. It is made possible thanks to the Muon Telescope Detector upgrade completed in 2014 at STAR. Among the various probes used to study the existence and properties of the Quark Gluon Plasma (QGP) created in relativistic heavy-ion collisions, quarkonia are considered a unique one. A fundamental feature of the QGP is that partons, instead of hadrons, are the relevant degrees of freedom of the system. Quarkonia are expected to exist as hadrons in the QGP until the Debye radius, which is inversely proportional to the medium temperature, becomes smaller than the quarkonium size. Under such conditions, the color-screening effect would lead to dissociation of the quarkonia in the medium. This can be observed experimentally through measurements of $J/\psi$ suppression in heavy-ion collisions with respect to that in p+p collisions. Such a suppression is believed to be unambiguous evidence of the QGP formation.

The figure below shows the $J/\psi$ RAA, used to quantify the suppression level, as a function of centrality for pT > 0.15 GeV/c and pT > 5 GeV/c in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV. At both low and high pT, the $J/\psi$ RAA is seen to decrease from peripheral to central collisions, consistent with increasing hot medium effects.

Figure: $J/\psi$ RAA as a function of centrality above 0.15 (left) and 5 (right) GeV/c in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV, compared to those for Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV.

For high-pT $J/\psi$, where the cold nuclear matter effects and the regeneration contribution are expected to be minimal, the $J/\psi$ production in 0-10% central collisions is suppressed by a factor of 3.1, providing strong evidence for the color-screening effect in a deconfined medium. Compared to similar measurements carried out for Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV, the low-pT $J/\psi$'s are much more suppressed in central and semi-central collisions at RHIC than at the LHC, likely due to the smaller charm quark production cross-section and thus smaller regeneration contribution at RHIC. On the Other hand, there is a hint that the high-pT $J/\psi$ RAA is systematically higher at RHIC for semi-central bins. This could be because the temperature of the medium created at the LHC is higher than that at RHIC, leading to a higher dissociation rate. The new results presented in this publication could help constrain model calculations and deepen our understanding of the QGP properties.

Posted October 24, 2019

STAR focus: Measurements of the transverse-momentum-dependent cross sections of $J/\psi$ production at mid-rapidity in proton+proton collisions at $\sqrt{s}$ = 510 and 500 GeV

The STAR Collaboration has recently published "Measurements of the transverse-momentum-dependent cross sections of $J/\psi$ production at mid-rapidity in proton+proton collisions at $\sqrt{s}$ = 510 and 500 GeV with the STAR detector" in Physical Review D 100, 052009 (2019).

The $J/\psi$ meson is a subatomic particle, a bound state of charm and anticharm quarks, which was discovered in 1974 at Brookhaven National Laboratory (BNL) and Standford Linear Accelerator Center. However, the $J/\psi$ production mechanism is still not yet fully understood after several decades of the discovery. Therefore, it is important to continue confronting the models that incorporate the most current understanding with new data.

Relativistic Heavy-Ion Collider (RHIC) at BNL provides a unique collision energy to study the detailed production mechanism of the $J/\psi$. In addition, in 2013-2014 the STAR detector has implemented a new subdetector dedicated for muon detection, the Muon Telescope Detector (M$ and this provides us an opportunity to probe the low-pT region of the $J/\psi$ production.

This publication presents the $J/\psi$ meson production cross sections in proton+proton collisions at center-of-mass energies of 510 and 500 GeV using the μ+μ- and e+e- decay channels to probe low-pT and high-pT regions, respectively.

FIG: (a) The $J/\psi$ differential full production cross sections as a function of $p_{T}^{J/\psi}$ in proton+proton collisions at $\sqrt{s}$ = 510 and 500 GeV measured through the $\mu^{+}\mu^{-}$ (blue stars) and $e^{+}e^{-}$ decay channels (red circles). The shaded region around the data points denotes the polarization envelope and the green curve is the estimation of the B-hadron feed-down from FONLL.
(b, c, d) Ratios of data and different model calculations to the Levy fit function.

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Posted October 24, 2019

STAR focus: STAR uses weak bosons to probe the spin structure of the proton

The STAR Collaboration has recently published, “Measurement of the longitudinal spin asymmetries for weak boson production in proton-proton collisions at √s = 510 GeV,” in Physical Review D 99, 051102(R).

This paper reports on measurements of the parity-violating asymmetry in the production of positrons and electrons from decays of weak bosons from collisions with one of the proton beams polarized longitudinally. In 510 GeV center-of-mass proton-proton collisions at RHIC, W+ bosons are produced primarily in the interactions of up quarks and down antiquarks, whereas W- bosons originate from down quarks and up antiquarks. The spin-asymmetry measurements of the decay positrons thus provide sensitivity to the up quark and down antiquark helicities in the proton, whereas the decay electrons do so for the down quark and up antiquark helicities. Combined, they make it possible to delineate the light quark and antiquark polarizations in the proton by flavor.

Left: Longitudinal single-spin asymmetries, AL, for W± production as a function of the positron or electron pseudorapidity, ηe, for the combined STAR 2011, 2012 and 2013 data samples for 25 < ET < 50GeV (points) in comparison to theory expectations (curves and bands). Right: The difference of the light sea-quark polarizations as a function of x at a scale of Q2 = 10(GeV/c)2. The green band shows the NNPDFpol1.1 results and the blue hatched band shows the corresponding distribution after the STAR 2013 W± data are included by reweighting.

These measurements provided one of the two initial motivations for the spin-physics program at RHIC and the data, shown in the left panel of the figure above, are the final data from STAR on this topic. Shown are the results from previously published data obtained in 2011 and 2012 combined with new results from the dedicated RHIC run in 2013. As seen from the right panel, the data have now reached a level of precision that makes it possible, for the first time, to conclude that the polarization of up antiquarks is larger than that of the down antiquarks.

Posted March 27, 2019

BNL News

STAR focus: Precise Measurements of the Open Charm Meson Prodution at STAR

The STAR Collaboration has recently published “Centrality and transverse momentum dependence of D0-meson production at mid-rapidity in Au + Au collisions at √sNN = 200 GeV,” in Physical Review C 99, 034908 (2019).

This paper presents a new measurement of D0 meson production enabled by the Heavy Flavor Tracker (HFT) high-resolution silicon detector system in Au + Au collisions at √sNN = 200 GeV. Compared to the previous measurement with the TPC only, the new data contain greatly improved precision which allows us to systematically investigate the D0 meson production in wide centrality and transverse momentum regions. The new improved data are expected to further constrain our understanding of the charm medium interaction as well as to better determine the medium transport parameters together with the previous publication on D0 elliptic flow measurement.

Left: D0 RCP for different centrality classes with the 40–60% spectrum as the reference compared to that of other light and strange mesons (π, Κs, and φ) as well as the model calculations. Right: Integrated D0 cross section per nucleon-nucleon collision at mid-rapidity for pT > 0 (a) and pT > 4 GeV/c (b) as a function of centrality Npart. The green boxes on the data points depict the overall normalization uncertainties in p + p and Au + Au data respectively.

The nuclear modification factors RCP of D0 meson are shown in the left figure, and show significantly suppression at high pT and the suppression level is comparable to that of light hadrons at pT > 5 GeV/c, indicates that charm quarks lose significant energy when traversing through the hot QCD medium. Right figure shows the pT-integrated D0 production cross section per nucleon-nucleon collisions in Au + Au collisions together with the measurement in p + p. The full-pT integrated cross section in Au + Au collisions seem to be smaller than that in p + p collisions by 1.5σ, indicating that cold nuclear matter effects (CNM) and/or hadronization through quark coalescence may play an important role in Au + Au collisions. While for pT > 4 GeV/c, it shows a clear decreasing trend from peripheral to mid-central and central collisions.

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Posted March 27, 2019

STAR focus: Azimuthal harmonics in small and large collision systems at RHIC top energies

The STAR Collaboration has recently published, Azimuthal Harmonics in Small and Large Collision Systems at RHIC Top Energies, in Physical Review Letter, 122, 172301(2019).

This publication reports and compares recent integral and differential measurements (obtained with the STAR TPC), of the flow harmonics ($v_n, n=1-3$) for charged hadrons produced in U+U collisions ($\sqrt{s_{{NN}}}$ = 193 GeV) and Au+Au, Cu+Au, Cu+Cu, d+Au, and p+Au collisions ($\sqrt{s_{{NN}}}$ = 200 GeV). The measurements for these disparate collision-systems, allow systematic variations of the initial-state eccentricity and its fluctuations, as well as the size of the produced fireball at approximately the same collision energy. All are expected to influence the magnitude of $v_n$. The comparisons between the measurements for these small, medium and large collision systems, give unique insights into the role of final-state interactions as the collision-system size is varied. Similarly, the measurements provide more discerning constraints which can aid extraction of the temperature-dependent specific shear viscosity of the hot and dense media created in the collisions.

Two-particle azimuthal correlation functions (a-f) and four-particle cumulants (g) for pT-integrated track pairs. Results are shown for U+U (a) collisions (193 GeV) and Au+Au (b), Cu+Au (c), Cu+Cu (d), d+Au (e) and p+Au (f) collisions (200 GeV) for the same charged particle multiplicity $N_{ch}$. Panel (g) shows the four-particle second-order cumulant vs. $N_{ch}$, obtained with the three sub-events method from the same data sets.

The measurements exploit both the two- and multi-particle correlation techniques to extract $v_n$ as a function of the transverse momentum ($p_T$) and mean charged particle multiplicity $N_{ch}$. The first figure shows representative correlation functions (a-f) and four particle cumulants (g) which accentuate the qualitative similarity between the charged particle azimuthal distributions obtained for the full range of collision-system sizes. Further quantitative study reveals that for a fixed value of $N_{ch}$, the $v^{even}_{1}$ and $v_{3}$ coefficients are essentially independent of the colliding species, indicating that for a given $N_{ch}$, the fluctuation-driven initial-state eccentricities, $\varepsilon_{1}$ and $\varepsilon_{3}$, are system independent. By contrast, the $v_{2}$ coefficients indicate sizeable variations (for fixed $N_{ch}$) with the colliding species, showing the strong collision-system dependence of the shape-driven eccentricity $\varepsilon_{2}.$

Posted May 31, 2018

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STAR focus: Collision Energy Dependence of pt Correlations in Au+Au Collisions at RHIC

The STAR Collaboration recently has had the paper “Collision-energy dependence of pt correlations in Au+Au collisions at energies available at the BNL Relativistic Heavy Ion Collider” has published in Phys. Rev. C 99, 044918.

The study of event-by-event correlations and fluctuations in global quantities can provide insight into the properties of the hot and dense matter created in Au+Au collisions at RHIC. Correlations of transverse momentum, pt, have been proposed as a measure of thermalization and as a probe for the critical point of QCD. A detailed study of the of dependence of two-particle pt correlations on collision energy and centrality may demonstrate the effects of thermalization. If the matter produced in collisions at RHIC passes through the QMD critical point, the fluctuations are predicted to increase with respect to a baseline of uncorrelated emission. A possible signature of the critical point could be non-monotonic behavior of the two-particle correlations as a function of the collision energy in central collisions.

This paper reports two-particle transverse-momentum correlations from Au+Au collisions taken during the RHIC Beam Energy Scan at center of mass energies ranging from 7.7 GeV to 200 GeV. These measurements are compared to previous measurements from the CERES Collaboration at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization.

Left: The relative dynamical correlation for 7.7 GeV and 200 GeV Au+Au collisions compare with similar results fron 2.76 TeV Pb+Pb collision. The dashed line represents a fit to the data at 200 GeV given by 22.3%/(Npart)1/2. Statistical and systematic errors are shown. Right: The relative dynamical correlation for Au+Au collisions as a function of collision energy for the 0-5% centrality bin along with results for Pb+Pb collisions from ALICE along with UrQMD calculations and results from Boltzmann-Langevin model calculations. The solid line is drawn to guide the eye. Statistical and systematic errors are shown.

Posted April 29, 2018

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STAR focus: STAR uses photons to probe the structure of gold nuclei

The STAR Collaboration has recently published “Coherent diffractive photoproduction of ρ0 mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider,” in Physical Review C 96, 054904 (2017).

This paper reports on a special type of heavy-ion interaction, where the ions do not physically collide, but interact via a long-range electromagnetic interaction, whereby photons emitted by one nucleus probe the structure of the other nucleus. The photons come from the electric and magnetic fields carried by the highly charged nuclei. The electric fields radiate radially outward, while magnetic fields circle the ion’s trajectory. The two fields are perpendicular, just like those of a photon, and they can be treated as such.

In the reaction considered here, the photon may be thought of as briefly fluctuating to a quark-antiquark pair, as allowed by the Heisenberg uncertainty principle. Quark-antiquark pairs are mesons; this photon fluctuation acts like a meson with the same quantum numbers (spin one and negative parity) as the photon. These virtual (short-lived) mesons can scatter from the target nucleus, and emerge as real mesons.

Left: The cross-section as a function of t, the squared momentum transfer to the nucleus. The dips and peaks are a diffraction pattern, akin to the pattern made by a 2-slit interferometer. ‘XnXn’ and ‘1n1n’ are two different STAR data samples.The inset shows the distribution for very small momentum transfers. Right: The two-dimensional Fourier transform of the left panel, showing the density of the interaction sites in the nucleus, as a function of transverse distance from its center. This is a map of where the mesons interacted in the target. Although there is considerable systematic uncertainty (the blue region) near the center of the target, the edges of the nuclei are well defined.

The photons scatter equally from protons and neutrons. But, we can’t tell which proton or neutron an individual meson scattered from. In quantum mechanics, we add the amplitudes to scatter from each target meson. The amplitude is a complex number with a phase which depends on the meson momentum and the position of the target nucleon. By studying how the scattering probability varies with the momentum transfer to the nucleus, we can image the matter distribution in the target. The left panel shows the scattering probability as a function of the square of the momentum transfer (‘t’) for two different STAR data samples. The dips are due to diffraction, like the fringes seen in the classic two-slit diffraction pattern, but with a circular target.

The right panel show the two-dimensional Fourier-Bessel (Henckel) transform of the left panel, mapping the interaction density within the target. The transform converts a function of momentum to a function of position. The FWHM of the distribution is 12.34 ± 0.24 fm. Because of nuclear shadowing, this is not just the nuclear density distribution; shadowing will alter the distribution from that of the density of a gold nucleus; these corrections may also alter the apparent size of the nucleus. Unlike electron scattering measurements, this analysis is sensitive to both protons and neutrons.

Posted Jan. 9, 2018

STAR focus: A new approach to jet quenching measurements

The STAR Collaboration has recently published a paper, Phys. Rev. C 96, 024905, presenting a novel approach to measurements of jet quenching, one of the most important ways to study the Quark-Gluon Plasma (QGP) generated in nuclear collisions at RHIC and the LHC.

High energy collisions generate "jets", which are correlated sprays of particles arising from the decay of energetic quarks and gluons. In heavy ion physics, jets provide self-generated tomographic probes of the QGP; they are produced in the collisions itself and interact with the surrounding matter before flying off to be observed in the detectors. This interaction between a jet and the QGP modifies jet properties dramatically relative to those in vacuum (“jet quenching”), and has produced some of the most striking measurements of the QGP. Such jet measurements are challenging, however. Jet quenching was initially discovered by STAR and PHENIX, in Au+Au collisions at RHIC, by studying distributions of single high-momentum particles and their correlations, which are indirect jet messengers.

Left Figure: Measured jet rate (signal plus background) in head-on Au+Au collisions (red points) and the "mixed event" background (grey distribution). Right Figure: Azimuthal deflection of jets recoiling from a trigger particle (data points), and a calculation including QCD effects but not scattering in the QGP (red curve).

The new STAR paper utilizes the distribution of charged-particle jets recoiling from a high momentum single-hadron trigger to study jet quenching. The huge backgrounds underlying the reconstructed jet signal in Au+Au collisions are measured using a sophisticated event-mixing technique. The contribution of uncorrelated background is then corrected "statistically", i.e. on the measured jet spectrum averaged over the entire ensemble of events, rather than attempting to correct for background on an event-by-event basis. This statistical-correction method enables jet measurements at RHIC over the complete range of jet momenta - including very low momentum - in all collision systems, for large jet-cone radius. This approach enables qualitatively new ways of studying jet quenching.

Posted Sep. 12, 2017

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STAR focus: Global Λ hyperon polarization in nuclear collisions

STAR has recently reported the first observation of global polarization of Lambda hyperons in heavy ion collisions. The discovery has been published in Nature 548, 62 (2017) as a cover story.

Due to the parity-violating nature of their weak decay, Lambdas reveal the direction of their spin by preferentially emitting the daughter proton along that direction. The average spin direction of a population of Lambdas is the polarization. Lambdas at midrapidity were topologically reconstructed in the STAR TPC, and the Beam-Beam Counters (BBC) at forward and backward rapidity were used to estimate the direction of the total angular momentum of the collision. We discovered that the polarization direction of the Lambdas was correlated at the level of several percent with the direction of the system angular momentum in non-central collisions at √sNN=7.7-32 GeV.

It has been well-established that the hot system created at midrapidity in the system may be considered a fluid, and hydrodynamic calculations relate the polarization of emitted particles is directly related to the vorticity - the curl of the flow field - of the fluid. Using this relation, we estimate that the curl of the fluid created at RHIC is about 9×1021 s-1, 14 orders of magnitude higher than any fluid ever observed. Previous results have established the system at RHIC to be the hottest and the least viscous (relative to entropy density) fluid ever created. Our new result adds another record - collisions at RHIC produce the most vortical fluid.

This first view of the rotational substructure of the fluid at RHIC represents an entirely new direction in hot QCD research. It has generated considerable theoretical activity in the field, and may have important connections with the Chiral Magnetic and Chiral Vortical Effects (CME and CVE). With increased statistics, there may even be the opportunity to probe the magnetic field produced in heavy ion collisions by measuring the difference in polarization of Lambda and AntiLambda hyperons. Such studies are planned for the future.

Posted Aug. 16, 2017

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BNL News

STAR focus: Evidence for Charm Quark Thermalization at RHIC

STAR has recently published its first paper to Phys. Rev. Letters on measurements enabled by the Heavy Flavor Tracker (HFT) high-resolution silicon detector system. The results are the first from a detector based on Monolithic Active Pixel Sensor (MAPS) technology in a collider environment and are the first measurements of D0 elliptic flow, v2, in Au+Au collisions at √sNN = 200 GeV.

Heavy flavor quarks, due to their large masses, are considered to offer unique information about QGP dynamics in heavy-ion collisions. A measurement of heavy flavor hadron v2, particularly in the low-to-intermediate pT region, will provide us a better understanding of medium thermalization, and can help quantitatively measure the heavy quark diffusion coefficient – one of the intrinsic transport parameters of the QGP.

Figure 1. Left: The Heavy Flavor Tracker (HFT) system which consists of one layer of Silicon Strip Detector (SSD), one layer of Intermediate Silicon Tracker (IST), and two layers of PiXeL detector (PXL). Right: Pointing resolution in the transverse plane as a function of particle momentum (or transverse momentum) at mid-rapidity from experiments at RHIC and the LHC.

The HFT consists of three subsystems: one layer of Silicon Strip Detector (SSD), one layer of Intermediate Silicon Tracker (IST) and two layers of silicon PiXeL (PXL) detectors. The HFT-PXL detector is the first application of the MAPS technology in a collider experiment. Its unique features include fine pixel size and thin material budget which provides superior track pointing resolution for charged particles over a broad momentum range. The HFT was designed for precision measurements of charmed hadron production via topological reconstruction of displaced vertices in heavy-ion collisions. The HFT was installed and taken physics data during RHIC Runs 2014-2016. The dataset used in the PRL was about 1.1B minimum-bias-triggered Au+Au 200 GeV events taken in 2014. Fig. 1 (right) shows the track pointing resolution in the transverse plane as a function of particle momentum (or transverse momentum) at mid-rapidity from experiments at RHIC and the LHC.

Figure 2. Left: v2 normalized by the number-of-constituent-quarks (nq) as a function of transverse kinetic energy (also normalized by nq) for D0 mesons and light hadrons in 10-40% central Au+Au collisions at 200 GeV. Right: v2 as a function of pT for D0 in 0-80% Au+Au collisions at 200 GeV compared to various model calculations.

Fig. 2 (left) shows the v2 normalized by the number-of-constituent-quark (nq) vs. the transverse kinetic energy (also normalized by nq) for D0 mesons from this measurement and other light hadron results. With unprecedented precision, the result shows that D0 v2 follows the same trend as light hadrons with this scaling. In the low pT region, this indicates a clear mass ordering for light hadrons and D0 mesons. In the intermediate pT region, the magnitude of the D-meson v2 is the same as light mesons. This result suggests that charm quarks have gained a similar amount of collectivity in these collisions as light mesons.

Fig. 2 (right) shows the D-meson v2 compared to various theoretical model calculations. One interesting observation is that the measured D-meson v2 can be well-described by a 3D viscous hydrodynamic model calculation, which indicates that charm quarks may have reached local thermal equilibrium. The precision of the current data allows us to distinguish different models but there are non-trivial differences between different models that need to be settled. One important physics goal, for the future, is to constrain the temperature dependence of the heavy quark diffusion coefficient parameter via joint investigations between theorists and experimentalists.

The D0 v2 results, together with other heavy flavor results from STAR (e.g. the enhancement observed in the Ds and ΛC production in mid-central Au+Au collisions as well as the charm/bottom-separated single-electron RAA measurements) have been reported and highlighted in the recent Quark Matter 2017 conference in Chicago in February [see Guannan's contribution to STAR Newsletter February 2017 edition]. These measurements strongly suggest that charm quarks may have reached thermalization in the Au+Au collisions at RHIC energy.

The paper was made possible with significant contributions from colleagues at BNL, CCNU, KSU, LBNL, MIT, Purdue, SINAP, UIC, USTC, and UT Austin, with critical support from the STAR operation and computing teams as well as the paper's GPC.

Posted Jun. 30, 2017

STAR focus: Jet-like correlations with direct-photon and neutral-pion triggers at √sNN = 200 GeV

The STAR experiment recently published “Jet-like correlations with direct-photon and neutral-pion triggers at √sNN = 200 GeV” in . Physics Letters B 760 (2016) 689.

Direct photons are produced during the early stage of a heavy-ion collision, through QCD processes such as quark-gluon Compton scattering and quark-antiquark pair annihilation (at leading order). In these processes the transverse momentum of the trigger photon approximates the initial transverse momentum of the recoil parton. The away-side parton (resulting in a spray of collimated hadrons called a “jet”) is expected to lose energy while traversing the medium. Jet-like charged-hadron yields on the recoil side of the trigger photon are calculated from the azimuthal angular correlation functions. The suppression of these jet-like yields in central Au+Au collisions is then quantified by comparing to the per-trigger yields measured in p+p collisions, denoting the ratio of integrated yields IAA

It is also compelling to compare the suppression of jet-like yields on the recoil side of direct-photon triggers with the suppression of jet-like yields on the recoil side of neutral-pion (or hadron) triggers. Differences in the suppression are expected from two effects. 1) There is a trigger bias for neutral pions (since they are themselves subject to medium interaction and energy loss) to be from the surface of the medium, maximizing the path length of the recoil parton through the medium; whereas direct-photon triggers can originate from anywhere within the medium (since the direct-photon mean-free-path is much larger than the size of the medium). 2) The recoil side of direct-photon triggers is dominated by quark jets, while the recoil of neutral-pion triggers can be either quark or gluon jets. Both of these effects naively should result in a larger suppression, on average, for the recoil jet-like yields associated with neutral-pion triggers than those associated with direct-photon triggers. One would expect to get information about both the path-length and the color-factor dependence of parton energy loss through the comparison.

Left: The IAA for direct-photon and neutral-pion triggers are plotted as a function of zT. The points for IAA for direct-photon triggers are shifted by +0.03 in zT for visibility. Right: IAA for direct-photon triggers as a function of transverse momentum of the jet-like associated hadrons. The vertical lines represent statistical errors and the vertical extent of the boxes represents systematic errors. The curves represent different energy-loss models.

The IAA for direct-photon (red) and neutral-pion (blue) triggers are plotted as a function of zT = pTassoc/pTtrig or the ratio of transverse momentum carried by the away-side hadron relative to that of the trigger) in the left panel of the figure. The results suggest that for jet-like associated hadrons, with transverse momentum greater than 1.2 GeV/c, the suppression factor is similar for direct-photon and neutral-pion triggers, within measurement uncertainties. The expected effects due to differences in path length and color-factor are not observed, within uncertainties, within our kinematic range. There is a hint of less suppression (for both types of triggers) at low zT, but this effect is more significant when IAA is plotted as a function of the transverse momentum of the jet-like associated hadrons pTassoc (shown in the right panel of the figure).

In contrast to these results, PHENIX (Phys. Rev. Lett. 111 (2013) 032301) has measured an enhancement in jet-like yields (IAA > 1), at large angles, for zT = 0.1-0.25. In the PHENIX measurement, the trigger photon has transverse momentum 5-9 GeV/c, and the associated hadrons have transverse momentum as low as 0.5 GeV/c (compared to the 1.2 GeV/c lower cut in the STAR measurement). Our measurement, in comparison with the PHENIX result, has led to the important conclusion that the modified fragmentation function is not a universal function of zT.

Posted Nov. 3, 2016

Previous STAR Focus Features

STAR focus: Measurement of interaction between antiprotons

The STAR Collaboration has for the first time directly measured the interaction between antiprotons, as presented in a recent publication Nature 527, 345–348(2015).

The interaction between two antinucleons is the basic interaction that binds the antinucleons into antinuclei, and this has not been directly measured previously. By applying a technique similar to Hanbury Brown and Twiss intensity interferometry, the force between two antiprotons at short range is found to be attractive. In addition, two key parameters that characterize the corresponding strong interaction are reported, namely, the scattering length and the effective range. The measured parameters are consistent within errors with the corresponding values for proton–proton interactions. The results provide direct information on the interaction between two antiprotons, one of the simplest systems of antinucleons, and so are fundamental to understanding the structure of more complex antinuclei and their properties. Of equal importance, one aspect of the current measurement is a test of matter–antimatter symmetry, more formally known as CPT—a fundamental symmetry of physical laws under the simultaneous transformations of charge conjugation (C), parity transformation (P) and time reversal (T).

Left figure: Correlation functions and their ratio. Correlation functions for proton–proton pairs (top) and antiproton–antiproton pairs (middle). The ratio of the former to the latter is shown in the bottom panel. Errors are statistical only. Cinclusive(k*), are plotted as solid lines, and the term 1 + xpp[Cpp(k*; Rpp) − 1] is shown as dashed lines.

Lower right figure: d0 versus f0 for (anti)nucleon-(anti)nucleon interactions.

The singlet s-wave scattering length (f0) and the effective range (d0) for the antiproton–antiproton interaction (red star) is plotted together with the s-wave scattering parameters for other nucleon–nucleon interactions.

Posted Nov. 5, 2015

BNL News Release

STAR focus: Precision Measurement of the Longitudinal Double-Spin Asymmetry for Inclusive Jet Production in Polarized Proton Collisions at √s = 200 GeV

A long-standing puzzle in quantum chromodynamics asks how the intrinsic spins and orbital angular momenta of the quarks, antiquarks, and gluons sum to give the proton spin of ħ/2. Polarized deep-inelastic lepton scattering (DIS) experiments have shown that the quark and antiquark spins account for less than one-third of the total. The DIS measurements provide only very weak constraints on the gluon spin contribution to the proton spin, and essentially nothing is known experimentally about the orbital angular momentum contribution. The ability of polarized proton collisions to probe gluon polarization was a primary motivation for establishing the RHIC spin program.

In a recent publication, Phys. Rev. Lett. 115, 092002 (2015), the STAR Collaboration presents the first experimental evidence for positive gluon polarization in the Bjorken-x region x > 0.05. The paper, which is highlighted as a PRL Editors’ Suggestion, presents a measurement of the longitudinal double-spin asymmetry, ALL, for midrapidity inclusive jet production in 200 GeV pp collisions. The data, which were recorded in 2009, provide nearly a 20-fold increase in the event statistics compared to previous STAR inclusive jet ALL measurements, and the analysis features improved jet reconstruction and correction techniques to reduce systematic uncertainties.

The measured inclusive jet ALL vs. parton jet pT is compared to predictions from several recent NLO global analyses of the polarized parton distributions. The error bars are statistical. The gray boxes show the size of the systematic uncertainties. The uncertainty bands associated with the NLO global analyses, which are of the order of or larger than the spread among the model calculations, are omitted for clarity.

The figure shows the results compared to predictions from several recent next-to-leading-order (NLO) global analyses of DIS, semi-inclusive DIS, and previous RHIC pp data. The results draw a narrow road through the uncertainty bands associated with those previous NLO global analyses, leading to substantially tighter constraints on gluon polarization than were provided by the previous world data. Very recently, these results have been included in new NLO global analyses by the DSSV and NNPDF groups. Both groups find that the gluon polarization in the Bjorken-x region x > 0.05 differs from zero by over 3σ, and contributes approximately 40% of the total proton spin.

Posted Aug. 27, 2015

STAR focus: Observation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions

The STAR collaboration has found supporting evidence for what’s called a “chiral magnetic wave” rippling through the soup of quark-gluon plasma created in RHIC’s energetic particle smashups, as presented in a recent publication, Phys. Rev. Lett. 114,252302 (2015).

In the picture with a chiral magnetic wave, the strong magnetic field created by the passing-by of spectators, with the presence of finite density of electric charge, causes chiral charges to separate along the axis of the magnetic field – a process called Chiral Separation Effect (CSE). The chiral separation acts like a seed that, in turn, causes particles with different electric charges to separate – in a process described as Chiral Magnetic Effect (CME). The evidence supporting the existence of the latter has been previously presented by STAR and hotly debated. The CSE and CME trigger each other and the process goes on like a wave. In the end, these two effects together will push negative particles into the equator and positive particles to the poles.

To look for this effect, STAR measured the collective motion of positively and negatively charged pions, the most abundant particles produced in RHIC collisions. It is found that the collective elliptic flow of negatively charged pions—their tendency to flow out along the equator—was enhanced, while the elliptic flow of the positive pions was suppressed, resulting in a higher abundance of positive particles at the poles. Importantly, the difference in elliptic flow between positive and negative pions increased with the net charge density produced in RHIC collisions. This is exactly what is expected from calculations using the theory predicting the existence of the chiral magnetic wave. The results hold out for almost all energies at which a quark-gluon plasma is believed to be created at RHIC, and, so far, no other model can explain them.

Left: Positive (negative) pion v2 linearly decreases (increases) with the increasing charge assymetry, which is expected by a CMW model. Right: The slope parameter, r, as a function of centrality for Au+Au collisions at 200 GeV. Also shown is the UrQMD simulation, and the calculation with CMW with different duration times. See paper for explanation for grey bands and cross-hatched band.

Posted June 27, 2015

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BNL Feature Story

STAR focus: ΛΛ Correlation Function in Au+Au collisions at √sNN = 200 GeV

The STAR Collaboration published in Physical Review Letters, Phys. Rev. Lett. 114, 022301 (2015), the first high statistics measurement of ΛΛ correlation function in Au+Au collisions at √sNN = 200 GeV and it is also highlighted as PRL editors' suggestion. This research pioneered the venue of using RHIC as a hyperon factory to investigate hyperon-hyperon interactions. The STAR measurement can provide precious data for the understanding of hyperon-hyperon interaction which is an important input to various baryon-baryon interaction potential model as well as for the study of equation of state for neutron stars. The ΛΛ interaction is also closely related to the existence of the H dibaryon, one of the most searched for exotic hadrons in nuclear collisions.

Left Fig. The combined ΛΛ and anti-Λ anti-Λ correlation function for 0-80% centrality Au+Au collisions at √sNN = 200 GeV. Curves correspond to fits using the Lednicky and Lyuboshitz analytical model with and without a residual correlation term. The dotted line corresponds to Fermi statistics with a source size of 3.13 fm. The shaded band corresponds to the systematic error.

The Lednicky and Lyuboshitz analytical model is used to fit the experimental correlation function and the resulting parameters suggest that the strength of the ΛΛ interaction is weak. The measured ΛΛ correlation shows deviation from unity as well as the free correlation expected from Fermi statistics. The correlation measurement also allowed extraction of an upper limit for possible production of the H dibaryon in Au+Au collisions at 200 GeV which is significantly lower than theoretical predictions based on coalescence calculations.

Posted Jan. 28, 2015

STAR focus: Electroweak bosons provide new probe of proton spin structure

The proton is composed of two up and one down flavor quarks that are bound together by the strong force. The strong force is mediated by gluons which in turn can spawn virtual quark and antiquark pairs known as sea quarks. Since the masses of the up and down (anti)quarks are small and nearly identical, it was expected that their number densities would also be very similar. However, experiments in the 1990s observed a significant asymmetry in the number of down and up antiquarks in the proton. Theories that successfully explained this asymmetry inspired predictions of even larger asymmetries in the polarized sector, generating renewed interest in determining the spin orientation of the proton’s antiquarks. The data from the STAR experiment, reported in this paper, Phys. Rev. Lett. 113, 072301 (2014), provide new constraints on these antiquark polarizations and shed light on the origin of the proton’s sea quarks.

Left Fig. The asymmetry in the production cross section for W± bosons in proton collisions with a positive (negative) proton beam helicity, σ+-), defined as AL=(σ+ - σ-)/(σ+ + σ-) is shown as a function of the W boson’s decay lepton pseudorapidity. The filled(open) points correspond to the W+(W-) asymmetries, in comparison to predictions from different polarized parton distribution functions.

In this measurement, the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) studied longitudinally polarized proton collisions at a center of mass energy of 0.5 TeV. W bosons are produced in these collisions by the annihilation of a quark and an antiquark. The quark flavors in the interaction can be determined by the charge of the W boson produced. The analysis by the STAR collaboration reports a significant asymmetry in the number of W bosons produced with a positive vs. negative helicity proton beam (shown in the figure above), and the data prefer a sizable, positive up flavor antiquark polarization in the kinematic range covered at RHIC.

Posted Aug. 13, 2014

STAR focus: Energy Dependence of Moments of Net-proton Multiplicity Distributions at RHIC

In the phase diagram of Quantum Chromodynamics (QCD–theory of strong interactions), it is conjectured on the basis of theoretical calculations that there will be a critical point (CP) at high temperature and non-zero baryonic chemical potential region. Experimental confirmation of the QCD Critical Point will be an excellent test of QCD theory in the non-perturbative region and the milestone of exploring the QCD phase diagram. This is one of the main goals of the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Due to the high sensitivity to the correlation length of dynamical systems and directly connected to the susceptibility in the Lattice QCD, higher moments of net-proton distributions have been applied to search for the QCD Critical Point in the heavy-ion collision experiment at STAR. The results were recently published in Phys. Rev. Lett. 112, 032302 (2014).

In this article, we report the beam energy (√s = 7.7 - 200 GeV) and collision centrality dependence of the mean (M), standard deviation (σ), skewness (S), and kurtosis (κ) of the net-proton multiplicity distributions in Au + Au collisions. The measurements are carried out by the STAR experiment at mid-rapidity (|y|< 0.5) in the first phase of the BES at RHIC.

Left Fig. The products of moments (Sσ and κσ2) in 0-5% Au+Au collisions are found to have values significantly below the Skellam expectation and have largest significance of deviations (the difference between data and Skellam baseline divided by the errors) for 19.6 and 27 GeV, with values of 3.2 and 3.4 for κσ2 and 4.5 and 5.6 for Sσ, respectively. These observables are not reproduced by either a non-CP transport model or by a hadron resonance model. The measurements are reasonably described by assuming independent production of proton and anti-protons, indicating that there are no apparent correlations between the protons and anti-protons for the observable presented. However at the lower beam energies, the net-proton measurements are dominated by the shape of the proton distributions only.

In summary, measurements of the higher moments of the net-proton distributions in Au+Au collisions over a wide range of μB have been presented to search for a possible CP. For √s > 39 GeV, Sσ and κσ2 values are similar for central, peripheral Au+Au collisions and p+p collisions. Deviations for both Sσ and κσ2 from HRG and Skellam expectations are observed for √s < 27 GeV. Higher statistics data for √s < 19.6 GeV will help to quantitatively understand the energy dependence of Sσ and κσ2. The conclusions on the existence of CP can be made only after comparison to QCD calculations with CP behavior which include the dynamics associated with heavy-ion collisions.

Posted Jan. 25, 2014

STAR focus: Observation of an Energy-Dependent Difference in Elliptic Flow between Particles and Antiparticles in Relativistic Heavy Ion Collisions

We present elliptic flow (v2) measurements for identified particles at mid-rapidity in Au+Au collisions, measured by the STAR experiment in the Beam Energy Scan at RHIC at √sNN= 7.7-62.4 GeV. A beam-energy dependent difference of the values of v2 between particles and corresponding anti-particles was observed. The difference increases with decreasing beam energy and is larger for baryons compared to mesons. This implies that, at lower energies, particles and anti-particles are not consistent with the universal number-of-constituent-quark (NCQ) scaling of v2 - one of the evidences in support of the sQGP formation at √sNN = 200 GeV. The results were recently published in Phys. Rev. Lett. 110, 142301 (2013).

Left figure: The difference in v2 between particles X and their corresponding anti-particles X (see legend) as a function of √sNN for 0-80% central Au+Au collisions. The dashed lines in the plot are fits with a power-law function. The error bars depict the combined statistical and systematic errors. The difference in the v2 values for particles and anti-particles increases with decreasing beam energy. The energy dependence in shape and magnitude is similar for all baryons. Baryons show a larger difference compared to mesons.

The v2(mT-m0) and possible NCQ scaling were investigated for particles and anti-particles separately, as shown in the lower figure. The baryons and mesons are clearly separated for √sNN = 62.4 GeV at (mT-m0) > 1 GeV/c2. While the effect is present for particles at √sNN = 11.5 GeV, no such separation is observed for the anti-particles at this energy in the measured (mT-m0) range up to 2 GeV/c2 . The lower panels of the figure depict the difference of the baryon v2 relative to a fit to the meson v2 data with the pions excluded from the fit. The anti-particles at √sNN = 11.5 GeV show a smaller difference compared to the particles.

In summary, the first observation of a beam-energy dependent difference in v2(pT) between particles and corresponding anti-particles for minimum bias √sNN = 7.7-62.4 GeV Au+Au collisions at mid-rapidity is reported. The difference increases with decreasing beam energy. It is apparent that, at the lower energies, particles and anti-particles are no longer consistent with the single NCQ scaling that was observed for √sNN = 200 GeV. However, for the group of particles the NCQ scaling holds within ±10% while for the group of anti-particles the difference between baryon and meson v2 continues to decrease to lower energies.

Posted Apr. 16, 2013

STAR focus: Identified hadron compositions in p+p and Au+Au collisions at high transverse momenta at √sNN = 200 GeV

We report identified particle pT spectra at mid-rapidity up to 15 GeV/c from p+p and Au+Au collisions at √sNN = 200 GeV. The NLO pQCD models describe the π± spectra but fail to reproduce the K and p(pbar) spectra at high pT. The measured anti-particle to particle ratios are observed to decrease with increasing pT. This reflects differences in scattering contributions to the production of particles and anti-particles at RHIC. At pT ≥ 8 GeV/c, a common suppression pattern is observed for different particle species. Incorporating our p+p data in generating the flavor separated fragmentation functions in the same kinematic range will provide new inputs and insights into the mechanisms of jet quenching in heavy ion collisions. These results have been published at Phys. Rev. Lett. 108, 072302 (2012).

Left: Yield ratios π-+, pbar/p, K-/K+, p/π+, pbar/π-, and K±, K0S± versus pT in p+p collisions, and nominal NLO calculations with AKK and DSS fragmentation functions without theoretical uncertainties. The open squares in panels (d) and (e) are the p/π+ and pbar/π- ratios in central Au+Au collisions with updated uncertainties at high pT, and all other data points are from p+p collisions. Bars and boxes (bands) represent statistical and systematic uncertainties, respectively.

Right: (a) RAA of K±+p(pbar), K0S, ρ0, and π± in central Au+Au collisions as a function of pT. The curves are the calculations for K0S RAAwith and without jet conversion in medium. Bars and boxes (bands) represent statistical and systematic uncertainties, respectively. The height of the band at unity represents the normalization uncertainty. (b) The ratios of RAA[K±+p(pbar), ρ0] to RAA±) and RAA(K-+pbar) to RAA(K++p). The boxes and shaded bands represent the systematic uncertainties for RAA0)/RAA±) and RAA[K±+p(pbar)]/RAA±), respectively. The systematic uncertainties for RAA(K-+pbar)/RAA(K++p) are 2%-12% and left off for clarity.

Read more... | Posted Mar. 20, 2012

star focus: Highlights from the STAR paper: Observation of the antimatter helium-4 nucleus
Scientists have discovered the heaviest antimatter nucleus: antihelium-4, which contains two antiprotons and two antineutrons. The new discovery is published online by Nature
High-energy nuclear collisions create an energy density similar to that of the universe microseconds after the Big Bang. In both cases, matter and antimatter are formed with comparable abundance. Thus, a high energy accelerator of heavy nuclei is an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus (4He),which known as the anti-α (α), has not been observed before. Although the α particle was identified a century ago by Rutherford and is present in cosmic radiation at the 10% level. The STAR Collaboration reports the observation of the antimatter helium-4 nucleus, the heaviest observed antinucleus. In total 18 4He counts were detected in 109 recorded Au+Au collisions at center-of-mass energies of 200 GeV and 62 GeV per nucleon-nucleon pair. The yield is consistent with expectations from thermodynamic and coalescent nucleosynthesis models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of 4He in cosmic radiation.

Left: The top two panels show the <dE/dx> in units of multiples of σdE/dx, nσdE/dx , of negatively charged particles (first panel) and positively charged particles (second panel) as a function of mass measured by the TOF system. The masses of 3He (3He) and 4He (4He) are indicated by the vertical lines at 2.81 GeV/c2 and 3.73 GeV/c2, respectively. The horizontal line marks the position of zero deviation from the expected value of <dE/dx> (nσdE/dx = 0) for 4He (4He). The rectangular boxes highlight areas for 4He (4He) selections : −2 < nσdE/dx < 3 and 3.35 GeV/c2 < mass < 4.04 GeV/c2 (corresponding to a ±3σ window in mass). The bottom panel shows a projection of entries in the upper two panels onto the mass axis for particles in the window of −2 < σdE/dx < 3. The combined measurements of energy loss and the time of flight allow a clean identification to be made in a sample of 0.5 × 1012 tracks from 109 Au+Au collisions.

Right: Differential invariant yields as a function of baryon number B, evaluated at pT /|B| = 0.875 GeV/c, in central 200 GeV Au+Au collisions. Yields for (anti)tritons (3H and 3H) lie close to the positions for 3He and 3He, but are not included here because of poorer identification of (anti)tritons. The lines represent fits with the exponential formula ∝ e−r|B| for positive and negative particles separately, where r is the production reduction factor. Analysis details of yields other than 4He (4He) have been presented elsewhere. Errors are statistical only. Systematic errors are smaller than the symbol size, and are not plotted.

Read more... | Posted April 25, 2011

star focus: Highlights from the STAR paper: Measurement of the parity-violating longitudinal single-spin asymmetry for W± boson production in polarized proton-proton collisions at √s = 500GeV
Accepted by Physical Review Letters

High energy polarized p + p collisions at √s = 200-500 GeV at RHIC provide a unique way to probe the proton spin structure and dynamics using hard scattering processes. The data taking period in 2009 of polarized p + p collisions at √s = 500 GeV opens a new era in the study of the spin-flavor structure of the proton based on the production of W±. W± bosons are produced predominantly through u + d (u + d) collisions and can be detected through their leptonic decay. The production of W bosons in polarized proton collisions allows for the observation of purely weak interactions, giving rise to large, parity-violating, longitudinal single-spin asymmetries. We report the first measurement of the parity violating single-spin asymmetries for midrapidity decay positrons and electrons from W+ and W- boson production in longitudinally polarized proton-proton collisions at √s = 500 GeV by the STAR experiment at RHIC. The measured asymmetries, AW+L = -0.27 ± 0.10 (stat.) ± 0.02 (syst.) ± 0.03 (norm.) and AW-L = 0.14 ± 0.19 (stat.) ± 0.02 (syst.) ± 0.01 (norm.), are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized DIS measurements.

At midrapidity, W± production probes a combination of the polarization of the u and d (d and u) quarks, and AW+(-)L is expected to be negative (positive). The measured AW+L is indeed negative at the 2.7 sigma level, which is a direct consequence of the positive u quark polarization. The central value of AW-L is positive as expected with a larger statistical uncertainty at the 0.7 sigma level. Our AL results are consistent with predictions using polarized quark and antiquark PDFs constrained by inclusive and semi-inclusive pDIS measurements, as expected from the universality of polarized PDFs. Future high-statistics measurements at midrapidity together with measurements at forward and backward pseudorapidities will focus on constraining the polarization of d and u quarks. The Run 11 data set will allow to expand the first W measurement at midrapidity to higher precision. The installation of the Forward GEM Tracker in summer 2011 provides the needed extension of the tracking capability in front of the STAR Endcap Electromagnetic Calorimeter.
Read more... | Posted January 5, 2011

star focus: Baryon Number Fluctuations to look for the QCD Critical Point
Highlights from the STAR paper: Higher Moments of Net-proton Multiplicity Distributions at RHIC Published in Physical Review Letters 105 (2010) 022302

A recent paper from the STAR Collaboration published in Physical Review Letters proposes using the higher moments of net-proton multiplicity distributions produced in high energy heavy-ion collisions as an observable for locating the QCD Critical Point. It has been shown that a careful choice of the products of the moments of the net-proton distributions form observables that can be related to the ratios of various order baryon number susceptibilities computed in QCD (basic theory of strong interactions) calculations. Thus the measurements provide a way for comparisons of heavy-ion collision data to first principle QCD calculations on lattice. Since susceptibilities diverge at critical point, these products of moments of net-proton distributions are also expected to take up larger values at the critical point. Thus the measurements reported provide a unique and new observable to search for landmark QCD critical point in QCD phase diagram of Temperature vs. Baryon chemical potential,in high energy heavy-ion collisions.

The measurements (product of kurtosis times the variance of net-proton distribution is shown in the figure) carried out at three different beam energies have been used to rule out the presence of QCD critical point below 200 MeV baryon chemical potential in the QCD phase plane. In high energy heavy-ion collisions the moments of net-protons, related to baryon number susceptibilities, have been shown to be independent of the system volume. QCD calculations on lattice have shown such a case happens when the system undergoes a cross over transition between hadronic and quark-gluon phases. In the near future these measurements (as indicated by the arrows at the bottom of the figure) will be carried out at varying collision energies or baryon chemical potential at the Relativistic Heavy Ion Collider to locate the QCD critical point. This is one of the physics goals of the RHIC Beam Energy Scan Program, moving towards that direction the STAR experiment has collected a good data set at beam energies of 7.7, 11.5 and 39 GeV this year.
Read more... | Posted August 20, 2010

star focus: Observation of an Antimatter Hypernucleus
Scientists report discovery of heaviest known antinucleus and first antinucleus containing an anti-strange quark, laying the first stake in a new frontier of physics

Nuclear collisions recreate conditions in the universe microseconds after the Big Bang. Only a very small fraction of the emitted fragments are light nuclei, but these states are of fundamental interest. We report the observation of antihypertritons—comprised of an antiproton, antineutron, and antilambda hyperon—produced by colliding gold nuclei at high energy. Our analysis yields 70 ± 17 antihypertritons (Formula) and 157 ± 30 hypertritons (Formula). The measured yields of Formula (Formula) and 3He (3Formula) are similar, suggesting an equilibrium in coordinate and momentum space populations of up, down, and strange quarks and antiquarks, unlike the pattern observed at lower collision energies. The production and properties of antinuclei, and nuclei containing strange quarks, have implications spanning nuclear/particle physics, astrophysics, and cosmology.
Read more... | Posted July 22, 2010

star focus: Long range rapidity correlations
Highlights from the STAR papers: Long range rapidity correlations and jet production in high energy nuclear collisions and Growth of Long Range Forward-Backward Multiplicity Correlations with Centrality in Au+Au Collisions at sqrt(sNN) = 200 GeV . Submitted for publication to Physical Review C and Physical Review Letters respectively.

The STAR experiment has now reported two interesting results on long range correlations in rapidity. One of the experimental observation is from a correlation study in azimthal angle and pseudorapidity for produced charged hadrons with respect to a particle with larger transverse momentum. Such studies revealed a jet-like correlation at small pair phase space separation (in azimuth and pseudorapidity - near side) which seems to be unmodified in central Au+Au collisions relative to d+Au and a significant correlated yield in central Au+Au collisions at large pair separation in pseudorapidity (the RIDGE). The ridge is observed in Au+Au collisions and not observed in d+Au collisions (See figures).

Read more... | Posted Sep 5, 2009

star focus: Photon multiplicity measurements at forward rapidity
Highlights from the STAR paper: Center of mass energy and system-size dependence of photon production at forward rapidity at RHIC. Submitted for publication to Physics Letters B.

Several interesting features of the dependence of particle density in pseudorapidity have been observed in Au+Au collisions from the experiments at the Relativistic Heavy-Ion Collider (RHIC). Particle production is found to follow a unique, collision energy independent, longitudinal scaling in p+p and d+Au, as well as in heavy-ion collisions. Such longitudinal scaling is also found to be independent of collision centrality for photons. The total charged particle multiplicity (integrated over the full pseudorapidity range) per average number of participating nucleon (< Npart >) pair is found to be independent of collision centrality by PHOBOS experiment. However, at mid-rapidity (|eta| <1), the PHENIX experiment showed that charged particle multiplicity per < Npart > is observed to increase from peripheral to central collisions. The charged particle production scales with a combination of < Npart > and average number of binary collisions < Nbin >. These clearly indicates that the mechanism of particle production could be different in different pseudorapidity regions. It is believed that the scaling of particle multiplicity with < Npart > indicates the dominance of soft processes in particle production, whereas scaling with average number of binary collisions (< Nbin >) indicates the onset of hard processes (pQCD jets).

Read more... | Posted Jul 5, 2009

star focus: J/Psi production at high transverse momentum
Highlights from the STAR paper: J/psi production at high transverse momentum in p+p and Cu+Cu collisions at sqrt(sNN) = 200 GeV Submitted for publication to Physical Review Letters.

Suppression of the c-cbar bound state J/Psi meson production in relativistic heavy-ion collisions arising from J/Psi dissociation due to screening of the c-cbar binding potential in the deconfined medium has been proposed as a signature of Quark-Gluon Plasma (QGP) formation. Measurements of high transverse momentum J/Psi production in A+A collisions relative to p+p collisions (Nuclear modification factor) can tell us which of the following physical scenario is possible:

(a) If nuclear modification factor is less than one it could be due partonic energy loss in dense matter, here the J/Psi formation then likely proceeds through a channel carrying color.

Read more... | Posted Apr 8, 2009

star focus: K/pi Fluctuations at Relativistic Energies
Highlights from the STAR paper: K/pi Fluctuations at Relativistic Energies . Submitted for publication to Physical Review Letters.

Strangeness enhancement has been predicted to be one of the important signatures of the formation of the quark gluon plasma (QGP). The study of dynamic fluctuations in event-by-event K/pi ratio may produce information concerning QCD phase transitions and may lead to the observation of the critical point in the QCD phase diagram. Studying fluctuations of particle ratios has few advantages, it has been argued that considering fluctuations of the multiplicity ratio eliminates the effect of volume fluctuations, further fluctuation of the particle ratio like K/pi could be sensitive to the particle numbers at chemical freeze-out and not at kinetic freeze-out. STAR experiment has recently reported the results on beam energy dependence of event-by-event K/pi ratio fluctuations at RHIC.

Read more... | Posted Feb 19, 2009

star focus: D* meson in jets
Highlights from the STAR paper: Measurement of D* Mesons in Jets from p+p Collisions at sqrt(s) = 200 GeV. Submitted for publication to Physical Review D Rapid Communications.

Studies by the ALEPH, L3 and OPAL Collaborations of the D* +/- meson content in jets show that the production from Z0 decays in e++e- collisions is dominated by D* mesons that carry large fractions of the jet momenta, consistent with the jets being produced from primary c (anti-)quarks. In pbar + p collisions at 630 GeV and 1.8 TeV, the UA1 and CDF Collaborations have observed D* +/- mesons in jets with transverse energies larger than 40 GeV. Their fractional momenta are found smaller, consistent with a different production mechanism in which the D* mesons originate from gluon splitting into c cbar pairs.

At top RHIC energy, heavy quarks can still be produced via gluon splitting. Perturbative QCD suggests that these contributions are small, and that the majority of the heavy quarks originate from gluon-gluon fusion. These expectations, however, have not until now been confronted with data at RHIC. The STAR experiment presents the first measurement of charged D* mesons in inclusive jets produced in p + p collisions at a center of mass energy of 200 GeV at RHIC which addresses the above issue. The charged D* andidates were identified through the decay sequence D*+ --> D0 pi+, D0 --> K-pi+ and its charge conjugate. The D*+ and D*- yields of 184 +/- 44 and 169 +/- 45 were obtained in inclusive jets with 11.5 GeV mean transverse energy.

Read more... | Posted Jan 24, 2009

star focus: Observation of Two-source Interference in STAR
Highlights from the STAR paper: Observation of Two-source Interference in the Photoproduction Reaction Au Au -> Au Au rho0. Submitted for publication to Physical Review Letters.

In ultra-peripheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a Rho0. The Rho0 production occurs in two well-separated (median impact parameters of 20 and 40 fermi for the cases considered here) nuclei, so the system forms a 2-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing Rho0 production. The produced Rho0s decay almost immediately at two well-separated points, so any interference must develop after the decay, and involve the pi(+) pi(-) final state. Since the pions go in different directions, this requires an entangled pi(+)pi(-) wave function which cannot be factorized into separate pi(+) and pi(-) wave functions; this is an example of the Einstein-Podolsky-Rosen paradox (for more details on this paradox look at reference below).

The figure shows the the uncorrected midrapidity minimum bias Au+Au 200 GeV dN/dt spectra as a function of t(perp) = (pT*pT). These data are compared simulations based with and without interference. The measured dN/dt spectrum is roughly exponential, but with a significant downturn for t(perp) < 0.0015 GeV*GeV, consistent with the predicted interference (dashed histogram). The no-interference histogram is almost exponential (solid histogram), dN/dt ~ exp (-kt(perp)), where k is related to the nuclear radius.

Read more... | Posted Jan 3, 2009

star focus: Phi Meson and Strangeness Enhancement at RHIC
Highlights from the STAR paper: Energy and system size dependence of phi-meson production in Cu+Cu and Au+Au collisions. Submitted for publication to Physics Letters B.

In a Quark-Gluon Plasma, thermal s and sbar quarks can be produced by gluon-gluon interactions. These interactions could occur very rapidly and the s-quark abundance would equilibriate. During hadronisation, the s and sbar quarks from the plasma coalesce to form phi-mesons. Production by this process is not suppressed as per the OZI (Okubo-Zweig-Izuka) rule. This, coupled with large abundances of strange quarks in the plasma, may lead to a dramatic increase in the production of phi-mesons and other strange hadrons relative to non-QGP p+p collisions.

Alternative ideas of canonical suppression of strangeness in small systems as a source of strangeness enhancement in high energy heavy-ion collisions have been proposed for other strange hadrons (e.g Kaon, Lambda, Cascade, Omega). The strangeness conservation laws require the production of an sbar-quark for each s-quark in the strong interaction. The main argument in such canonical models is that the energy and space time extensions in smaller systems may not be sufficiently large. This leads to a suppression of strange hadron production in small collision systems. These statistical models fit the data reasonably well. According to these models, strangeness enhancement in nucleus-nucleus collisions, relative to p+p collisions, should increase with the strange quark content of the hadrons. This enhancement is predicted to decrease with increasing beam energy.

Read more... | Posted Oct 29, 2008

star focus: STAR readiness for proposed Beam Energy Scan Program : Results from Au+Au collisions at 9.2 GeV
Highlights from recent data taken by STAR with the lowest beam energy collisions at RHIC - Au+Au collisions at 9.2 GeV. These were presented for the first time at the International Conference on Strangeness in Quark Matter, 2008, Beijing China.

One of the main aim of high energy heavy-ion collisions is to map the QCD phase diagram. The goal being to locate the QCD phase boundary (separating matter with hadronic degrees of freedom from matter with quark gluon degrees of freedom) and the QCD critical point (where the first order phase transition ends). The phase diagram is plotted as temperature versus baryon chemical potential. These quantities can be changed by varying the colliding beam energy to map the phase diagram. The temperature and baryon chemical potential can be measured from the produced particle spectra and ratios. Then one looks for signatures for different phases and for the QCD critical point. In addition STAR also would like to study the beam energy which corresponds to onset of several interesting observations seen at top RHIC energy (Au+Au 200 GeV) : Number of constituent quark scaling of elliptic flow parameter for produced hadrons, enhanced correlated yields at large delta_eta for delta_phi ~ 0 (Ridge) and the suppression of high transverse momentum hadron production in heavy ion collisions.

In order to achieve the above goals STAR has proposed a beam energy scan program at RHIC spanning beam energies from 5 GeV to 50 GeV. As a first step towards achieving this goal, recently STAR collected data from a test run for Au+Au collisions at 9.2 GeV. The events for this test run was collected at a rate of 0.7 Hz. The first results were presented at the SQM2008. Here we discuss only a small subset of the results.

Read more... | Posted Oct 9, 2008

star focus: Hadronic resonance measurements in d+Au collisions
Highlights from the STAR paper Hadronic resonance production in d+Au collisions at 200 GeV at RHIC accepted for publication in Physical Review C.

The particle identification capability of the Time Projection Chamber in STAR and its large acceptance enables us to measure many hadronic resonances produced in the high energy collisions. Resonances are strongly decaying particles with lifetimes x velocity of light that are of the order of the size of the hot and dense medium produced in heavy-ion collisions. The in-medium effects related to the high density and/or high temperature of the medium can modify the properties of short-lived resonances, such as their masses, widths, and even their spectral shapes. STAR experiment has recently reported the measurement of the following resonances for colliding beam energy of 200 GeV. These are reconstructed from their hadronic decay channels using invariant mass technique in d+Au collisions - rho(770), K*(892), Delta(1232)++, Sigma(1385), Lambda(1520).

One interesting feature was observed in the transverse mass distribution of these resonances measured at midrapidity. As shown in the figure, they seem to follow a generalized scaling in d+Au collisions between transverse mass range of 1 - 2 GeV/c2. Such a scaling could be envisaged within the idea of saturation of gluon density in the nucleus for high energy collisions. However such scaling has been observed in p+p collisions at ISR, SppbarS, RHIC energies. Also the resonances in d+Au collisions do not show any difference in the shape of the transverse mass distribution between baryons and mesons at higher transverse mass. Differences were earlier observed for non-resonant particles along baryon-meson lines.

Read more... | Posted Sep 20, 2008

star focus: Identified hadron measurements in STAR using the Time Projection Chamber
Highlights from the STAR paper Systematic Measurements of Identified Particle Spectra in p+p, d+Au and Au+Au Collisions from STAR submitted recently to Physical Review C.

STAR experiment has recently reported systematic measurements of identified particle spectra in pp, d+Au and Au+Au collisions. Along with reporting several interesting results for the above collision systems at different energies we have also presented in detail the particle identification procedure in STAR Time Projection Chamber and the various correction factors associated with the extraction of the yield and shape parameters for the transverse momentum spectra of produced hadrons.

In this focus article we present two results from this work. For Au+Au collisions,mean pT, which characterizes the slope of the transverse momentum spectra are found to increase significantly with increasing collision centrality or decreasing impact parameter of the collision. The trends are similar at 62.4 GeV, 130 GeV, and 200 GeV, and mean pT qualitatively agree with each other at the same dNch/dy. This suggests that the kinetic freeze-out properties in Au+Au collisions are rather energy independent for the measured collision energies.

Read more... | Posted Sep 6, 2008

star focus: Transverse Single Spin Asymmetries measurements in RHIC
Highlights from the STAR paper Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at sqrt(s)=200 GeV submitted recently to Physical Review Letters.

The production of particles with high transverse momentum from polarized proton collisions at high energies is sensitive to the quark and gluon spin structure of the proton.

One challenge to theory has been to understand the sizable azimuthal asymmetry of particles produced in collisions of transversely polarized protons, known as analyzing power (AN) or transverse single spin asymmetry (SSA) for inclusive pion production in polarized p + p collisions over a broad range of collision energies and in semi-inclusive deep inelastic scattering (SIDIS) from transversely polarized proton targets.

Spin-correlated transverse momentum dependent (TMD)distribution functions (Sivers effect), in conjunction with initial- or final-state color-charge interactions, can explain large AN. These functions describe parton orbital motion within the proton, and so are important to explore to understand the structure of the proton.

Read more... | Posted Aug 13, 2008

star focus: Beam-Energy and System-Size Dependence of Dynamical Net Charge at RHIC
Highlights from the STAR paper Beam-Energy and System-Size Dependence of Dynamical Net Charge submitted recently to Physical Review C.

Anomalous transverse momentum and net charge event-by-event fluctuations have been proposed as indicators of the formation of a quark gluon plasma (QGP) in high-energy heavy ion collisions. In particular, Koch et al. [1] have estimated that entropy conserving hadronization of a plasma of quarks and gluons should produce a final state characterized by a dramatic reduction of the net charge fluctuations relative to those observed in a hadron gas. Published STAR measurements indicate the fluctuations observed in Au + Au collision at center of mass energy of 130 GeV are little suppressed relative to those observed in p + p collisions [2]. STAR found the measured fluctuations in this collision system and energy are in qualitative accord with expectations based on hadron gas models. It is thus interesting to study the magnitude of the fluctuations as a function of the colliding system size by varying both collision centrality and colliding nuclei. There is also a possibility that final state interactions may partly wash out the expected suppression through collision and diffusion processes [3]. Best conditions to observe the predicted suppression may not be at 130 GeV. It is,therefore, of great interest to carry out a study of the system size, and beam energy dependence of the net charge fluctuations.

Read more... | Posted Jul 28, 2008

star focus: System-size independence of directed flow at RHIC
Highlights from the STAR paper System-size independence of directed flow at the Relativistic Heavy-Ion Collider submitted recently to Physical Review Letters.

Directed flow refers to collective sidewards deflection of particles and is characterized by a first-order harmonic (v1) of the Fourier expansion of particle's azimuthal distribution w.r.t. the reaction plane in heavy-ion collisions. STAR has recently submitted to Physical Review Letters multiple differential measurements of v1 for Au+Au and Cu+Cu collisions at center of mass energies of 200 and 62.4 GeV as a function of pseudorapidity (eta), transverse momentum, and collision centrality. We find that directed flow violates the "entropy-driven" multiplicity scaling which dominates all other soft observables. STAR has reported an intriguing new universal scaling of the phenomenon with collision centrality. Neither Boltzmann/cascade nor hydrodynamic models are able to explain the measured trends.

Read more... | Posted Jul 14, 2008

star focus: Charmed hadron production at low transverse momentum at RHIC
Highlights from the STAR paper Charmed hadron production at low transverse momentum in Au+Au collisions at RHIC submitted recently to Physical Review Letters.

Charm quarks are likely to be produced only in the early stages and can be a unique tool to probe the partonic matter created in relativistic heavy-ion collisions at RHIC energies. Studies of the number of binary collision (calculated using Glauber model) scaling of the total charm cross section from d+Au to Au+Au collisions can be used to test if charm production is exclusively at the initial impact of colliding heavy ions. The total charm production cross section is also an important input in models of J/Psi production via charm quark coalescence in a Quark Gluon Plasma.

Read more... | Posted Jun 27, 2008

star focus: indication of conical emission at RHIC
Highlights from the STAR paper Indications of Conical Emission of Charged Hadrons at RHIC submitted recently to Physical Review Letters.

Experimental observation of jet-quenching studies in STAR revealed: on the away side of a high transverse momentum (pt) trigger particle the correlated yield is strongly suppressed at pt > 2 GeV/c while at lower pt the yield is enhanced and the correlated hadrons appear to be partially equilibrated with the bulk medium and are broadly distributed in azimuth.

Read more... | Posted Jun 13, 2008

star focus: Half-Day Symposium: STAR Highlights and Future, Brookhaven National Laboratory, May 5th, 2008

At this event we celebrate Tim Hallman’s leadership as Spokesperson of the STAR experiment at RHIC for the past six years.  We chronicle the experiment’s success, and look forward towards its bright future.

Sponsored by the Department of Physics, Brookhaven National Laboratory

Posted Apr 25, 2008

star focus: jets in nuclear collisions
Part 1 of a series on STAR analysis topics

In high energy p+p collisions, the hard scattering of quarks and gluons early in the collision leads to the production of jets, narrow streams of particles that allow physicists to detect and understand the scattering. In nuclear collisions at RHIC, jets instead serve as a penetrating probe of the extremely dense nuclear matter formed in the collision. Comparing characteristics of jets in nuclear collisions to jets in p+p collisions has uncovered special properties of dense nuclear matter at RHIC.
Read more... | Posted Aug 3, 2006

star focus: beamtime!

STAR's preparations are underway for the 2006 RHIC beamtime, thanks to special funding for this year's experimental operations. This year's "beamtime" is the sixth annual RHIC run, with about 15 weeks of colliding polarized proton beams scheduled to begin in early March. STAR collaborators come to BNL during the experiment to serve weekly shifts as part of six-member shift teams, and each member has specific tasks to perform while on shift. More...
RHIC '06 operations plan | Posted Feb 2, 2006

star focus: quark matter 2005

The 18th International Conference on Nucleus Nucleus Collisions (Quark Matter 2005) was held in Budapest, Hungary, August 4-9. STAR presented new results on several fronts, summarized in two experimental summary talks on the conference's first day and a special focus talk on the last day. STAR contributed 15 parallel talks and many posters.
STAR at QM 2005 | Posted Aug 11, 2005

star focus: let's meet in warsaw

This summer, STAR will hold its semi-annual collaboration meeting in Warsaw, Poland, preceeding the Quark Matter 2005 conference in Hungary. The collaboration meeting will be hosted by Warsaw University of Technology (Politechnika Warszawska), a STAR member institute. Meetings before major conferences give collaboration members the chance to review results, share posters and practice talks. Meetings away from BNL also allow institutions to open their doors to fellow collaborators.
Read more... | Posted May 31, 2005

Central Square in Warsaw, Poland

star focus: rhic lessons

As part of a joint venture between BNL and JINR (Dubna) called the Online Science Classroom, a series of RHIC Lessons have been created by STAR Collaborators in Dubna. These lessons introduce many aspects of the research carried out by STAR, from the process of colliding different beams of particles to the challenges of studying the formation of a quark-gluon plasma. The lessons require Macromedia Flash Player.
RHIC Lessons | Posted March 5, 2005

star focus: graduate students

One of the primary missions of STAR is to provide rigorous training for graduate students, who account for much of the hands-on detector and analysis efforts for the experiment. As a result, STAR has produced an impressive number of graduate degrees over a wide range of thesis topics. Click here for a list of graduate thesis titles, with links to many thesis files.
Read more... | Posted January 21, 2005

star focus: physics results

Nearly four years have passed since STAR's first publication, on the observation of evidence for strong early expansion in Au+Au collisions at RHIC, appeared in Physical Review Letters. Since then, 23 other Physical Review Letters have followed, more than any nuclear physics experiment in history. A growing list of brief summaries of our publications is available on our website.
Physics results summaries | Posted November 19, 2004

star focus: star and the data grid

As the amount of data recorded by the STAR Experiment accumulates, the data storage and data processing needs grow as well. STAR is an active participant in the Particle Physics Data Grid, a project to manage and distribute data and data analysis tasks across a large network of computing facilities around the country.
Read more... | Posted September 24, 2004

star focus: regional meetings

STAR Collaborators comprise a community of scientists and technicians from 13 countries on four continents, and getting everyone together isn't always an easy task. That's one reason behind STAR Regional Meetings, smaller gatherings of STAR members a bit further from RHIC than the BNL physics building. Regional Meetings were held in China and Russia in 2003. Next up: Bhubaneswar, India, in October.
Read more... | Posted August 1, 2004

Mukteshvara Temple in Bhubaneswar

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