We report systematic measurements of bulk properties of the system created in Au+Au collisions at √sNN = 14.5 GeV recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The ...transverse momentum spectra of π±, K± and p(p¯) are studied at mid-rapidity (|y| < 0.1) for nine centrality intervals. The centrality, transverse momentum (pT), and pseudorapidity (η) 3 dependence of inclusive charged particle elliptic flow (v2), and rapidity-odd charged particles directed flow (v1) results near mid-rapidity are also presented. These measurements are compared with the published results from Au+Au collisions at other energies, and from Pb+Pb collisions at √sNN =2.76 TeV. The results at √sNN = 14.5 GeV show similar behavior as established at other energies and fit well in the energy dependence trend. These results are important as the 14.5 GeV energy fills the gap in μB, which is of the order of 100 MeV, between √sNN =11.5 and 19.6 GeV. Comparisons of the data with UrQMD and AMPT models show poor agreement in general.
We report new STAR measurements of the single-spin asymmetries AL for W+ and W− bosons produced in polarized proton-proton collisions at s=510 GeV as a function of the decay-positron and ...decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250 pb−1. The results are combined with previous results obtained with 86 pb−1. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton-proton data suggests a difference between the u¯ and d¯ quark helicity distributions for 0.05<x<0.25. In addition, we report new results for the double-spin asymmetries ALL for W±, as well as AL for Z/γ* production and subsequent decay into electron-positron pairs.
Here, we present measurements of three-particle correlations for various harmonics in Au+Au collisions at energies ranging from √sNN=7.7 to 200 GeV using the STAR detector. The quantity < ...cos(mΦ1+nΦ2–(m+n)Φ3) >, with Φ being the azimuthal angles of the particles is evaluated as a function of √sNN, collision centrality, transverse momentum, pT, pseudorapidity difference, Δη, and harmonics (m and n). These data provide detailed information on global event properties such as the three-dimensional structure of the initial overlap region, the expansion dynamics of the matter produced in the collisions, and the transport properties of the medium. A strong dependence on Δη is observed for most harmonic combinations, which is consistent with breaking of longitudinal boost invariance. An interesting energy dependence is observed when one of the harmonics m,n, or m+n is equal to two, for which the correlators are dominated by the two-particle correlations relative to the second-harmonic event plane. These measurements can be used to constrain models of heavy-ion collisions over a wide range of temperature and baryon chemical potential.
Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME)—an ...electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ( Δγ) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy (v2) . We report here differential measurements of the correlator as a function of the pair invariant mass (minv) in 20–50% centrality Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider. Strong resonance background contributions to Δγ are observed. At large minv where this background is significantly reduced, the Δγ value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the v2 background shape as a function of minv . We extract a v2-independent and minv -averaged signal Δγsig = (0.03 ± 0.06 ± 0.08 ) × 10 -4, or (2 ± 4 ± 5)% of the inclusive Δγ(minv > 0.4 GeV/c2) = ( 1.58 ± 0.02 ± 0.02 ) × 10-4, within pion pT = 0.2 –0.8 GeV/c and averaged over pseudorapidity ranges of -1 < η < - 0.05 and 0.05 < η < 1. This represents an upper limit of 0.23 × 10 -4 , or 15% of the inclusive result, at 95% confidence level for the minv -integrated CME contribution.
We report the first measurements of transverse single-spin asymmetries for inclusive jet and jet+π± production at midrapidity from transversely polarized proton-proton collisions at s=500 GeV. The ...data were collected in 2011 with the STAR detector sampled from 23 pb−1 integrated luminosity with an average beam polarization of 53%. Asymmetries are reported for jets with transverse momenta 6<pT<55 GeV/c and pseudorapidity |η|<1. Presented are measurements of the inclusive-jet azimuthal transverse single-spin asymmetry, sensitive to twist-3 initial-state quark-gluon correlators; the Collins asymmetry, sensitive to quark transversity coupled to the polarized Collins fragmentation function; and the first measurement of the “Collins-like” asymmetry, sensitive to linearly polarized gluons. Within the present statistical precision, inclusive-jet and Collins-like asymmetries are small, with the latter allowing the first experimental constraints on gluon linear polarization in a polarized proton. At higher values of jet transverse momenta, we observe the first nonzero Collins asymmetries in polarized-proton collisions, with a statistical significance of greater than 5σ. The results span a range of x similar to results from semi-inclusive deep-inelastic scattering but at much higher Q2. The Collins results enable tests of universality and factorization breaking in the transverse momentum-dependent formulation of perturbative quantum chromodynamics.
We present results from a harmonic decomposition of two-particle azimuthal correlations measured with the STAR detector in Au+Au collisions for energies ranging from sqrts_{NN}=7.7 to 200 GeV. The ...third harmonic v_{3}^{2}{2}=⟨cos3(ϕ_{1}-ϕ_{2})⟩, where ϕ_{1}-ϕ_{2} is the angular difference in azimuth, is studied as a function of the pseudorapidity difference between particle pairs Δη=η_{1}-η_{2}. Nonzero v_{3}^{2}{2} is directly related to the previously observed large-Δη narrow-Δϕ ridge correlations and has been shown in models to be sensitive to the existence of a low viscosity quark gluon plasma phase. For sufficiently central collisions, v_{3}^{2}{2} persist down to an energy of 7.7 GeV, suggesting that quark gluon plasma may be created even in these low energy collisions. In peripheral collisions at these low energies, however, v_{3}^{2}{2} is consistent with zero. When scaled by the pseudorapidity density of charged-particle multiplicity per participating nucleon pair, v_{3}^{2}{2} for central collisions shows a minimum near sqrts_{NN}=20 GeV.
Here we present high-precision measurements of elliptic, triangular, and quadrangular flow v2, v3, and v4, respectively, at midrapidity for identified hadrons π, p, K, φ, Ks, Λ as a function of ...centrality and transverse momentum in Au+Au collisions at the center-of-mass energy √sNN = 200 GeV. We observe similar vn trends between light and strange mesons which indicates that the heavier strange quarks flow as strongly as the lighter up and down quarks. The number-of-constituent-quark scaling for v2, v3, and v4 is found to hold within statistical uncertainty for 0–10%, 10–40%, and 40–80% collision centrality intervals. The results are compared to several viscous hydrodynamic calculations with varying initial conditions, and could serve as an additional constraint to the development of hydrodynamic models.
We present measurements of Ω and φ production at midrapidity from Au+Au collisions at nucleon-nucleon center-of-mass energies sNN=7.7, 11.5, 19.6, 27, and 39 GeV by the STAR experiment at the BNL ...Relativistic Heavy Ion Collider (RHIC). Motivated by the coalescence formation mechanism for these strange hadrons, we study the ratios of N(Ω-+Ω#x0002B;)/2N(φ). These ratios as a function of transverse momentum pT fall on a consistent trend at high collision energies, but start to show deviations in peripheral collisions at sNN=19.6, 27, and 39 GeV, and in central collisions at 11.5 GeV in the intermediate pT region of 2.4-3.6 GeV/c. We further evaluate empirically the strange quark pT distributions at hadronization by studying the Ω/φ ratios scaled by the number of constituent quarks (NCQ). The NCQ-scaled Ω/φ ratios show a suppression of strange quark production in central collisions at 11.5 GeV compared to sNN≥19.6 GeV. The shapes of the presumably thermal strange quark distributions in 0-60% most central collisions at 7.7 GeV show significant deviations from those in 0-10% most central collisions at higher energies. These features suggest that there is likely a change of the underlying strange quark dynamics in the transition from quark matter to hadronic matter at collision energies below 19.6 GeV.
We report J / I spectra for transverse momenta p T > 5 GeV / c at mid-rapidity in p + p and Au + Au collisions at s NN = 200 GeV . The inclusive J / I spectrum and the extracted B-hadron feed-down ...are compared to models incorporating different production mechanisms. We observe significant suppression of the J / I yields for p T > 5 GeV / c in 0a30% central Au + Au collisions relative to the p + p yield scaled by the number of binary nucleonanucleon collisions in Au + Au collisions. In 30a60% mid-central collisions, no such suppression is observed. The level of suppression is consistently less than that of high- p T pi +/- and low- p T J / I at RHIC and high- p T J / I at the LHC.
We report on the W and Z/γ∗ differential and total cross sections as well as the W+/W− and (W+ + W−)/(Z/γ∗) cross section ratios measured by the STAR experiment at RHIC in p + p collisions at √s = ...500 GeV and 510 GeV. The cross sections and their ratios are sensitive to quark and antiquark parton distribution functions. In particular, at leading order, the W cross section ratio is sensitive to the d/u ratio. These measurements were taken at high Q2 ∼ M2W, M2Z and can serve as input into global analyses to provide constraints on the sea quark distributions. The results presented here combine three STAR datasets from 2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb−1. We also assess the expected impact that our W+ / W− cross section ratios will have on various quark distributions, and find sensitivity to the u − d and d/u distributions.
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