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STAR's measurements of directed flow (v1) around midrapidity for π±, K±, K0S, p and p¯ in Au + Au collisions at $\sqrtsNN = 200$ GeV are presented. A negative v1(y) slope is observed for most of produced particles (π±, K±, K0S and p¯). The proton v1(y) slope is found to be much closer to zero compared to antiprotons. A sizable difference is seen between v1 of protons and antiprotons in 5-30% central collisions. The v1 excitation function is presented. Comparisons to model calculations (RQMD, UrQMD, AMPT, QGSM with parton recombination, and a hydrodynamics model with a tilted source) are made. Anti-flow alone cannot explain the centrality dependence of the difference between the v1(y) slopes of protons and antiprotons.
STAR's measurements of directed flow (v1) around midrapidity for π±, K±, K0S, p and p¯ in Au + Au collisions at $\sqrtsNN = 200$ GeV are presented. A negative v1(y) slope is observed for most of produced particles (π±, K±, K0S and p¯). In 5-30% central collisions a sizable difference is present between the v1(y) slope of protons and antiprotons, with the former being consistent with zero within errors. The v1 excitation function is presented. Comparisons to model calculations (RQMD, UrQMD, AMPT, QGSM with parton recombination, and a hydrodynamics model with a tilted source) are made. For those models which have calculations of v1 for both pions and protons, none of them can describe v1(y) for pions and protons simultaneously. The hydrodynamics model with a tilted source as currently implemented cannot explain the centrality dependence of the difference between the v1(y) slopes of protons and antiprotons.
The STAR Collaboration at the Relativistic Heavy Ion Collider presents measurements of 𝐽/𝜓→𝑒+𝑒− at midrapidity and high transverse momentum (𝑝𝑇>5 GeV/𝑐) in 𝑝+𝑝 and central Cu+Cu collisions at √𝑠𝑁𝑁=200 GeV. The inclusive 𝐽/𝜓 production cross section for Cu+Cu collisions is found to be consistent at high 𝑝𝑇 with the binary collision-scaled cross section for 𝑝+𝑝 collisions. At a confidence level of 97%, this is in contrast to a suppression of 𝐽/𝜓 production observed at lower 𝑝𝑇. Azimuthal correlations of 𝐽/𝜓 with charged hadrons in 𝑝+𝑝 collisions provide an estimate of the contribution of 𝐵-hadron decays to 𝐽/𝜓 production of 13%±5%.
We report a high precision measurement of the transverse single spin asymmetry AN at the center of mass energy √s=200 GeV in elastic proton–proton scattering by the STAR experiment at RHIC. The AN was measured in the four-momentum transfer squared t range 0.003⩽|t|⩽0.035 (GeV/c)2, the region of a significant interference between the electromagnetic and hadronic scattering amplitudes. The measured values of AN and its t-dependence are consistent with a vanishing hadronic spin-flip amplitude, thus providing strong constraints on the ratio of the single spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated by the Pomeron amplitude at this √s, we conclude that this measurement addresses the question about the presence of a hadronic spin flip due to the Pomeron exchange in polarized proton–proton elastic scattering.
Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at sqrt(s_NN)=130 GeV using the STAR TPC at RHIC. The elliptic flow signal, v_2, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.
Rapidity-odd directed flow measurements at midrapidity are presented for Λ, Λ¯, K±, K0s and ϕ at sNN−−−−√= 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions recorded by the STAR detector at the Relativistic Heavy Ion Collider. These measurements greatly expand the scope of data available to constrain models with differing prescriptions for the equation of state of quantum chromodynamics. Results show good sensitivity for testing a picture where flow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. The pattern of departure from a coalescence-inspired sum-rule can be a valuable new tool for probing the collision dynamics.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (pT) within −0.5<y<0 and 0.4<pT<2.0 GeV/c. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be C4/C2=−0.85±0.09 (stat.)±0.82 (syst.), which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (pT) within −0.5<y<0 and 0.4<pT<2.0 GeV/c. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be C4/C2=−0.85±0.09 (stat.)±0.82 (syst.), which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at sNN−−−√ = 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (pT) within −0.5<y<0 and 0.4<pT<2.0 GeV/c. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be C4/C2=−0.85±0.09 (stat.)±0.82 (syst.), which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.
We present the first measurements of charge-dependent correlations on angular difference variables η1 − η2 (pseudorapidity) and φ1 − φ2 (azimuth) for primary charged hadrons with transverse momentum 0.15 <= pt <= 2 GeV/c and |η| <= 1.3 from Au–Au collisions at √sNN = 130 GeV. We observe correlation structures not predicted by theory but consistent with evolution of hadron emission geometry with increasing centrality from one-dimensional fragmentation of color strings along the beam direction to an at least two-dimensional hadronization geometry along the beam and azimuth directions of a hadron-opaque bulk medium.