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

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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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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