The Relativistic Heavy Ion Collider (RHIC) is the world’s only polarized proton collider at high energies. One main goal of the RHIC SPIN program is to provide measurements with these unique data to gain deeper insight into the spin structure and dynamics of the proton.

The STAR experiment has collected several longitudinally polarized proton-proton collision data sets, mainly dedicated to study the polarized gluon distribution function (Δg(x)) of the proton. This function quantifies the contribution of gluons to the total spin of the proton. It can be accessed by measuring the longitudinal double-spin asymmetry (A_LL ) of inclusive jet and dijet production in proton-proton collisions.

In the recently published result “Longitudinal double-spin asymmetry for inclusive jet and dijet production in polarized proton collisions at √s= 510 GeV” in Phys. Rev. D 105 092011, we are providing the lastest STAR A_LL measurements for inclusive jets at midrapidity. These measurements are in agreement with previous STAR measurements and with predictions from current next-to-leading-order global analyses, as seen in the figure.

Figure: Inclusive jet A_LL versus x_T , compared to previous STAR results and evaluations from DSSV14 and NNPDFpol1.1 (with its uncertainty) global analyses. The vertical lines are statistical uncertainties. The boxes show the size of the estimated systematic uncertainties. Scale uncertainties from polarization (not shown) are ±6.5%, ±6.6%, ±6.4% and ±6.1% from 2009 to 2015, respectively.

Additionally, this work presents results for dijet production, which provides a better determination of the functional form of the polarized gluon distribution function. With a redesigned and optimized set of triggers, we were able to increase the statistics in the low dijet mass region by approximately an order of magnitude, which is critical to enable a controlled extrapolation of the polarized gluon distribution function in this gluon-rich region. These high precision measurements motivate the natural step forward for an Electron Ion Collider in order to study the gluon-rich region of the proton in even greater detail.

Posted May 23, 2022

Previous STAR Focus Features

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

Back-to-back dihadron azimuthal angle correlation has been proposed to be a sensitive probe to directly access the underlying gluon dynamics involved in hard scatterings. With a high gluon density at the initial state, the product of back-to-back dihadron modulation will be suppressed. It is predicted that the density of gluons per unit transverse area is larger in nuclei than in nucleons and is amplified by a factor of A^1/3 for a nucleus with mass number A; thus, nuclei provide a natural environment to study nonlinear gluon evolution.

The STAR Collaboration performed the measurements of back-to-back azimuthal correlations of di-π⁰s produced at forward pseudorapidities (2.6 < η < 4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass energy of 200 GeV. The results have been recently published in Phys. Rev. Lett. 129, 092501. We observe a clear suppression of the correlated yields of back-to-back π⁰ pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression is larger at smaller transverse momentum, which indicates lower x and Q² . The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale Q²_s on the mass number A. A linear scaling of the suppression with A^1/3 is observed with a slope of −0.09±01.

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

Posted August 22, 2022

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

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

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

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

Posted May 23, 2022

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

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

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

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

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

Posted May 29, 2022

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