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We report a systematic measurement of cumulants, Cn, for net-proton, proton and antiproton, and correlation functions, κn, for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at sNN−−−√ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The Cn and κn are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y, and transverse momentum, pT. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity (|y|< 0.5) and transverse momentum 0.4 < pT < 2.0 GeV/c, using the STAR detector at RHIC. We observe a non-monotonic energy dependence (sNN−−−√ = 7.7 -- 62.4 GeV) of the net-proton C4/C2 with the significance of 3.1σ for the 0-5\% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with sNN−−−√. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ2, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ4, of protons plays a role in determining the energy dependence of proton C4/C1 below 19.6 GeV, which cannot be solely understood by the negative values of κ2 for protons.
We report a systematic measurement of cumulants, Cn, for net-proton, proton and antiproton multiplicity distributions, and correlation functions, κn, for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at sNN−−−√ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The Cn and κn are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y, and transverse momentum, pT. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the BNL Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity (|y|< 0.5) and transverse momentum 0.4 < pT < 2.0 GeV/c, using the STAR detector at RHIC. We observe a non-monotonic energy dependence (sNN−−−√ = 7.7 -- 62.4 GeV) of the net-proton C4/C2 with the significance of 3.1σ for the 0-5\% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with sNN−−−√. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ2, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ4, of protons plays a role in determining the energy dependence of proton C4/C1 below 19.6 GeV, which cannot be understood by the effect of baryon number conservation.
The STAR Collaboration reports measurements of the transverse single-spin asymmetry (TSSA) of inclusive 𝜋0 at center-of-mass energies (√𝑠) of 200 GeV and 500 GeV in transversely polarized proton-proton collisions in the pseudo-rapidity region 2.7 to 4.0. The results at the two different energies show a continuous increase of the TSSA with Feynman-𝑥, and, when compared to previous measurements, no dependence on √𝑠 from 19.4 GeV to 500 GeV is found. To investigate the underlying physics leading to this large TSSA, different topologies have been studied. 𝜋0 with no nearby particles tend to have a higher TSSA than inclusive 𝜋0. The TSSA for inclusive electromagnetic jets, sensitive to the Sivers effect in the initial state, is substantially smaller, but shows the same behavior as the inclusive 𝜋0 asymmetry as a function of Feynman-𝑥. To investigate final-state effects, the Collins asymmetry of 𝜋0 inside electromagnetic jets has been measured. The Collins asymmetry is analyzed for its dependence on the 𝜋0 momentum transverse to the jet thrust axis and its dependence on the fraction of jet energy carried by the 𝜋0. The asymmetry was found to be small in each case for both center-of-mass energies. All the measurements are compared to QCD-based theoretical calculations for transverse-momentum-dependent parton distribution functions and fragmentation functions. Some discrepancies are found, which indicates new mechanisms might be involved.
Measurement of cold nuclear matter effects for inclusive J/ψ in p+Au collisions at √sNN = 200 GeV
(2022)
Measurement by the STAR experiment at RHIC of the cold nuclear matter (CNM) effects experienced by inclusive J/ψ at mid-rapidity in 0-100% p+Au collisions at √sNN = 200 GeV is presented. Such effects are quantified utilizing the nuclear modification factor, RpAu, obtained by taking a ratio of J/ψ yield in p+Au collisions to that in p+p collisions scaled by the number of binary nucleon-nucleon collisions. The differential J/ψ yield in both p+p and p+Au collisions is measured through the dimuon decay channel, taking advantage of the trigger capability provided by the Muon Telescope Detector in the RHIC 2015 run. Consequently, the J/ψ RpAu is derived within the transverse momentum (pT) range of 0 to 10 GeV/c. A suppression of approximately 30% is observed for pT < 2 GeV/c, while J/ψ RpAu becomes compatible with unity for pT greater than 3 GeV/c, indicating the J/ψ yield is minimally affected by the CNM effects at high pT. Comparison to a similar measurement from 0-20% central Au+Au collisions reveals that the observed strong J/ψ suppression above 3 GeV/c is mostly due to the hot medium effects, providing strong evidence for the formation of the quark-gluon plasma in these collisions. Several model calculations show qualitative agreement with the measured J/ψ RpAu, while their agreement with the J/ψ yields in p+p and p+Au collisions is worse.
We report on the measurements of directed flow v1 and elliptic flow v2 for hadrons (π±, K ±, K0 S , p, φ, Λ and ) from Au+Au collisions at √sN N = 3 GeV and v2 for (π±, K ±, p and p) at 27 and 54.4 GeV with the STAR experiment. While at the two higher energy midcentral collisions the numberof-constituent-quark (NCQ) scaling holds, at 3 GeV the v2 at midrapidity is negative for all hadrons and the NCQ scaling is absent. In addition, the v1 slopes at midrapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative v2 and positive v1 slope at 3 GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that the medium in such collisions is likely characterized by baryonic interactions.
In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in √sNN = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, v1 and v2, of light nuclei (d, t, 3He, 4He) produced in √sNN = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured v1 slopes of light nuclei at mid-rapidity. For the measured v2 magnitude, a strong rapidity dependence is observed. Unlike v2 at higher collision energies, the v2 values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.
We report the first multi-differential measurements of strange hadrons of K −, φ and − yields as well as the ratios of φ/K − and φ/− in Au+Au collisions at √sNN = 3 GeV with the STAR experiment fixed target configuration at RHIC. The φ mesons and − hyperons are measured through hadronic decay channels, φ → K + K − and Ξ− → Λπ−. Collision centrality and rapidity dependence of the transverse momentum spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the φ/K − and φ/− ratios while the result of canonical ensemble (CE) calculations reproduce φ/K −, with the correlation length rc ∼ 2.7 fm, and φ/−, rc ∼ 4.2 fm, for the 0-10% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3 GeV implies a rather different medium property at high baryon density.
We report the measurement of K∗0 meson at midrapidity (|y|< 1.0) in Au+Au collisions at sNN−−−√~=~7.7, 11.5, 14.5, 19.6, 27 and 39 GeV collected by the STAR experiment during the RHIC beam energy scan (BES) program. The transverse momentum spectra, yield, and average transverse momentum of K∗0 are presented as functions of collision centrality and beam energy. The K∗0/K yield ratios are presented for different collision centrality intervals and beam energies. The K∗0/K ratio in heavy-ion collisions are observed to be smaller than that in small system collisions (e+e and p+p). The K∗0/K ratio follows a similar centrality dependence to that observed in previous RHIC and LHC measurements. The data favor the scenario of the dominance of hadronic re-scattering over regeneration for K∗0 production in the hadronic phase of the medium.
We report a measurement of cumulants and correlation functions of event-by-event proton multiplicity distributions from fixed-target Au+Au collisions at sNN−−−√ = 3 GeV measured by the STAR experiment. Protons are identified within the rapidity (y) and transverse momentum (pT) region −0.9<y<0 and 0.4<pT<2.0 GeV/c in the center-of-mass frame. A systematic analysis of the proton cumulants and correlation functions up to sixth-order as well as the corresponding ratios as a function of the collision centrality, pT, and y are presented. The effect of pileup and initial volume fluctuations on these observables and the respective corrections are discussed in detail. The results are compared to calculations from the hadronic transport UrQMD model as well as a hydrodynamic model. In the most central 5\% collisions, the value of proton cumulant ratio C4/C2 is negative, drastically different from the values observed in Au+Au collisions at higher energies. Compared to model calculations including Lattice QCD, a hadronic transport model, and a hydrodynamic model, the strong suppression in the ratio of C4/C2 at 3 GeV Au+Au collisions indicates an energy regime dominated by hadronic interactions.
We report the first multi-differential measurements of strange hadrons of K−, ϕ and Ξ− yields as well as the ratios of ϕ/K− and ϕ/Ξ− in Au+Au collisions at sNN−−−√=3GeV with the STAR experiment fixed target configuration at RHIC. The ϕ mesons and Ξ− hyperons are measured through hadronic decay channels, ϕ→K+K− and Ξ−→Λπ−. Collision centrality and rapidity dependence of the transverse spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At the collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the ϕ/K− ratio while the result of canonical ensemble (CE) calculations reproduce well the ratios of ϕ/K−, with the correlation length rc∼2.7\,fm, and ϕ/Ξ−, rc∼4.2\,fm, for the 0-10\% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3GeV implies a rather different medium property at high baryon density.
We report the first multi-differential measurements of strange hadrons of K−, ϕ and Ξ− yields as well as the ratios of ϕ/K− and ϕ/Ξ− in Au+Au collisions at sNN−−−√=3GeV with the STAR experiment fixed target configuration at RHIC. The ϕ mesons and Ξ− hyperons are measured through hadronic decay channels, ϕ→K+K− and Ξ−→Λπ−. Collision centrality and rapidity dependence of the transverse momentum spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the ϕ/K− ratio while the result of canonical ensemble (CE) calculations reproduce well the ratios of ϕ/K−, with the correlation length rc∼2.7\,fm, and ϕ/Ξ−, rc∼4.2\,fm, for the 0-10\% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3GeV implies a rather different medium property at high baryon density.
We report the first multi-differential measurements of strange hadrons of K−, ϕ and Ξ− yields as well as the ratios of ϕ/K− and ϕ/Ξ− in Au+Au collisions at sNN−−−√=3GeV with the STAR experiment fixed target configuration at RHIC. The ϕ mesons and Ξ− hyperons are measured through hadronic decay channels, ϕ→K+K− and Ξ−→Λπ−. Collision centrality and rapidity dependence of the transverse momentum spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the ϕ/K− and ϕ/Ξ− ratios while the result of canonical ensemble (CE) calculations reproduce ϕ/K−, with the correlation length rc∼2.7\,fm, and ϕ/Ξ−, rc∼4.2\,fm, for the 0-10\% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3GeV implies a rather different medium property at high baryon density.
We report the first multi-differential measurements of strange hadrons of K−, ϕ and Ξ− yields as well as the ratios of ϕ/K− and ϕ/Ξ− in Au+Au collisions at sNN−−−√=3GeV with the STAR experiment fixed target configuration at RHIC. The ϕ mesons and Ξ− hyperons are measured through hadronic decay channels, ϕ→K+K− and Ξ−→Λπ−. Collision centrality and rapidity dependence of the transverse momentum spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the ϕ/K− and ϕ/Ξ− ratios while the result of canonical ensemble (CE) calculations reproduce ϕ/K−, with the correlation length rc∼2.7\,fm, and ϕ/Ξ−, rc∼4.2\,fm, for the 0-10\% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3GeV implies a rather different medium property at high baryon density.
The STAR Collaboration reports 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. 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 of back-to-back pairs as a function of event activity and transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale, Q2s, on the mass number, A. The suppression in high-activity p+Au collisions is consistent with theoretical predictions including gluon saturation effects.
The STAR Collaboration reports 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. 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 of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale, Q2s, on the mass number, A. A linear scaling of the suppression with A1/3 is observed with a slope of −0.09 ± 0.01.
We report measurements of the longitudinal double-spin asymmetry, ALL, for inclusive jet and dijet production in polarized proton-proton collisions at midrapidity and center-of-mass energy s√ = 510 GeV, using the high luminosity data sample collected by the STAR experiment in 2013. These measurements complement and improve the precision of previous STAR measurements at the same center-of-mass energy that probe the polarized gluon distribution function at partonic momentum fraction 0.015 ≲x≲ 0.25. The dijet asymmetries are separated into four jet-pair topologies, which provide further constraints on the x dependence of the polarized gluon distribution function. These measurements are in agreement with previous STAR measurements and with predictions from current next-to-leading order global analyses. They provide more precise data at low dijet invariant mass that will better constraint the shape of the polarized gluon distribution function of the proton.
We report measurements of the longitudinal double-spin asymmetry, ALL, for inclusive jet and dijet production in polarized proton-proton collisions at midrapidity and center-of-mass energy s√ = 510 GeV, using the high luminosity data sample collected by the STAR experiment in 2013. These measurements complement and improve the precision of previous STAR measurements at the same center-of-mass energy that probe the polarized gluon distribution function at partonic momentum fraction 0.015 ≲x≲ 0.25. The dijet asymmetries are separated into four jet-pair topologies, which provide further constraints on the x dependence of the polarized gluon distribution function. These measurements are in agreement with previous STAR measurements and with predictions from current next-to-leading order global analyses. They provide more precise data at low dijet invariant mass that will better constraint the shape of the polarized gluon distribution function of the proton.
Measurement of cold nuclear matter effects for inclusive J/ψ in p+Au collisions at √sNN = 200 GeV
(2021)
Measurement by the STAR experiment at RHIC of the cold nuclear matter (CNM) effects experienced by inclusive J/ψ at mid-rapidity in p+Au collisions at sNN−−−√ = 200 GeV is presented. Such effects are quantified utilizing the nuclear modification factor, RpAu, obtained by taking a ratio of J/ψ yield in p+Au collisions to that in p+p collisions scaled by the number of binary nucleon-nucleon collisions. The differential J/ψ yield in both p+p and p+Au collisions is measured through the dimuon decay channel, taking advantage of the trigger capability provided by the Muon Telescope Detector in the RHIC 2015 run. Consequently, the J/ψ RpAu is derived within the transverse momentum (pT) range of 0 to 10 GeV/c. A suppression of approximately 30% is observed for pT<2 GeV/c, while J/ψ RpAu becomes compatible with unity for pT greater than 3 GeV/c, indicating the J/ψ yield is minimally affected by the CNM effects at high pT. Comparison to a similar measurement from 0-20% central Au+Au collisions reveals that the observed strong J/ψ suppression above 3 Gev/c is mostly due to the hot medium effects, providing strong evidence for the formation of the quark-gluon plasma in these collisions. Several model calculations show qualitative agreement with the measured J/ψ RpAu, while their agreement with the J/ψ yield in p+p and p+Au collisions is worse.
Measurement of cold nuclear matter effects for inclusive J/ψ in p+Au collisions at √sNN = 200 GeV
(2022)
Measurement by the STAR experiment at RHIC of the cold nuclear matter (CNM) effects experienced by inclusive J/ψ at mid-rapidity in 0-100\% p+Au collisions at sNN−−−√ = 200 GeV is presented. Such effects are quantified utilizing the nuclear modification factor, RpAu, obtained by taking a ratio of J/ψ yield in p+Au collisions to that in p+p collisions scaled by the number of binary nucleon-nucleon collisions. The differential J/ψ yield in both p+p and p+Au collisions is measured through the dimuon decay channel, taking advantage of the trigger capability provided by the Muon Telescope Detector in the RHIC 2015 run. Consequently, the J/ψ RpAu is derived within the transverse momentum (pT) range of 0 to 10 GeV/c. A suppression of approximately 30% is observed for pT<2 GeV/c, while J/ψ RpAu becomes compatible with unity for pT greater than 3 GeV/c, indicating the J/ψ yield is minimally affected by the CNM effects at high pT. Comparison to a similar measurement from 0-20% central Au+Au collisions reveals that the observed strong J/ψ suppression above 3 Gev/c is mostly due to the hot medium effects, providing strong evidence for the formation of the quark-gluon plasma in these collisions. Several model calculations show qualitative agreement with the measured J/ψ RpAu, while their agreement with the J/ψ yields in p+p and p+Au collisions is worse.
Azimuthal anisotropy measurement of (multi-)strange hadrons in Au+Au collisions at √sNN = 54.4 GeV
(2023)
Azimuthal anisotropy of produced particles is one of the most important observables used to access the collective properties of the expanding medium created in relativistic heavy-ion collisions. In this paper, we present second (v2) and third (v3) order azimuthal anisotropies of K0S, ϕ, Λ, Ξ and Ω at mid-rapidity (|y|<1) in Au+Au collisions at sNN−−−√ = 54.4 GeV measured by the STAR detector. The v2 and v3 are measured as a function of transverse momentum and centrality. Their energy dependence is also studied. v3 is found to be more sensitive to the change in the center-of-mass energy than v2. Scaling by constituent quark number is found to hold for v2 within 10%. This observation could be evidence for the development of partonic collectivity in 54.4 GeV Au+Au collisions. Differences in v2 and v3 between baryons and anti-baryons are presented, and ratios of v3/v3/22 are studied and motivated by hydrodynamical calculations. The ratio of v2 of ϕ mesons to that of anti-protons (v2(ϕ)/v2(p¯)) shows centrality dependence at low transverse momentum, presumably resulting from the larger effects from hadronic interactions on anti-proton v2.