Beam-Energy Dependence of the Directed Flow of Protons, Antiprotons, and Pions in Au plus Au Collisions

Rapidity-odd directed flow (upsilon 1) measurements for charged pions, protons, and antiprotons near midrapidity (y = 0) are reported in root(S)(NN) = 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeVAu+Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider. At intermediate impact parameters, the proton and net-proton slope parameter d upsilon(1) = d upsilon(1)vertical bar (y=0) shows a minimum between 11.5 and 19.6 GeV. In addition, the net-proton d upsilon(1) = d upsilon(1)vertical bar (y=0) changes sign twice between 7.7 and 39 GeV. The proton and net-proton results qualitatively resemble predictions of a hydrodynamic model with a first-order phase transition from hadronic matter to deconfined matter, and differ from hadronic transport calculations

Measurement of Longitudinal Spin Asymmetries for Weak Boson Production in Polarized Proton-Proton Collisions at RHIC

We report measurements of single- and double-spin asymmetries for W-+/- and Z/gamma* boson production in longitudinally polarized p + p collisions at root s = 510 GeV by the STAR experiment at RHIC. The asymmetries for W-+/- were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the proton's polarized quark distributions at the scale of the W mass. The results are compared to theoretical predictions, constrained by polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range 0.05 < x < 0.2

J/psi production at low p(T) in Au plus Au and Cu plus Cu collisions at root s(NN)=200 GeV with the STAR detector

The J/psi p(T) spectrum and nuclear modification factor (R-AA) are reported for p(T) < 5 GeV/c and vertical bar y vertical bar < 1 from 0% to 60% central Au + Au and Cu + Cu collisions at root s(NN) = 200 GeV at STAR. A significant suppression of p(T) - integrated J/psi production is observed in central Au + Au events. The Cu + Cu data are consistent with no suppression, although the precision is limited by the available statistics. R-AA in Au + Au collisions exhibits a strong suppression at low transverse momentum and gradually increases with p(T). The data are compared to high-p(T) STAR results and previously published BNL Relativistic Heavy Ion Collider results. Comparing with model calculations, it is found that the invariant yields at low p(T) are significantly above hydrodynamic flow predictions but are consistent with models that include color screening and regeneration

Beam Energy Dependence of Moments of the Net-Charge Multiplicity Distributions in Au plus Au Collisions at RHIC

We report the first measurements of the moments-mean (M), variance (sigma(2)), skewness (S), and kurtosis (kappa)-of the net-charge multiplicity distributions at midrapidity in Au + Au collisions at seven energies, ranging from root s(NN) = 7.7 to 200 GeV, as a part of the Beam Energy Scan program at RHIC. The moments are related to the thermodynamic susceptibilities of net charge, and are sensitive to the location of the QCD critical point. We compare the products of the moments, sigma(2)/M, S sigma, and kappa sigma(2), with the expectations from Poisson and negative binomial distributions (NBDs). The S sigma values deviate from the Poisson baseline and are close to the NBD baseline, while the kappa sigma(2) values tend to lie between the two. Within the present uncertainties, our data do not show nonmonotonic behavior as a function of collision energy. These measurements provide a valuable tool to extract the freeze-out parameters in heavy-ion collisions by comparing with theoretical models

Jet-Hadron Correlations in root s(NN)=200 GeV p plus p and Central Au plus Au Collisions

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au + Au and p + p collisions at root s(NN) = 200 GeV in STAR are presented. The trigger jet population in Au + Au collisions is biased toward jets that have not interacted with the medium, allowing easier matching of jet energies between Au + Au and p + p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum (p(T)(assoc)) and enhanced at low p(T)(assoc) in 0%-20% central Au + Au collisions compared to p + p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions

Jet-Hadron Correlations in root s(NN)=200 GeV p plus p and Central Au plus Au Collisions

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au + Au and p + p collisions at root s(NN) = 200 GeV in STAR are presented. The trigger jet population in Au + Au collisions is biased toward jets that have not interacted with the medium, allowing easier matching of jet energies between Au + Au and p + p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum (p(T)(assoc)) and enhanced at low p(T)(assoc) in 0%-20% central Au + Au collisions compared to p + p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions

Measurements of dielectron production in Au plus Au collisions at root s(NN)=200 GeV from the STAR experiment

We report on measurements of dielectron (e(+) e(-)) production in Au + Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at BNL Relativistic Heavy Ion Collider. Systematic measurements of the dielectron yield as a function of transverse momentum (p(T)) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region (M-ee < 1 GeV / c(2)). This enhancement cannot be reproduced by the rho-meson vacuum spectral function. In minimum-bias collisions, in the invariant-mass range of 0.30-0.76 GeV / c(2), integrated over the full pT acceptance, the enhancement factor is 1.76 +/- 0.06 (stat.) +/- 0.26 (sys.) +/- 0.29 (cocktail). The enhancement factor exhibits weak centrality and pT dependence in STAR's accessible kinematic regions, while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 +/- 0.10. Models that assume an in-medium broadening of the rho-meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of omega-and phi-meson production through their e+ e(-) decay channel. These measurements show good agreement with Tsallis blast-wave model predictions, as well as, in the case of the phi meson, results through its K+ K- decay channel. In the intermediate invariant-mass region (1.1 < Mee < 3 GeV / c(2)), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed

Dielectron Mass Spectra from Au plus Au Collisions at root s(NN)=200 Ge V

We report the STAR measurements of dielectron (e(+)e(-)) production at midrapidity (vertical bar y(ee)vertical bar < 1) in Au + Au collisions at root s(NN) = 200 GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30-0.76 (rho-like), 0.76-0.80 (omega-like), and 0.98-1.05 (phi-like) GeV/c(2). The spectrum in the omega-like and phi-like regions can be well described by the hadronic cocktail simulation. In the rho-like region, however, the vacuum rho spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77 +/- 0.11(stat) +/- 0.24(syst) +/- 0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the rho meson. The excess yield in the rho-like region increases with the number of collision participants faster than the omega and phi yields. Theoretical models with broadened rho contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies

Centrality dependence of identified particle elliptic flow in relativistic heavy ion collisions at sNN=\sqrt{s_{NN}}= 7.7-62.4 GeV

Elliptic flow ($v_{2}$) values for identified particles at midrapidity in Au + Au collisions measured by the STAR experiment in the Beam Energy Scan at the Relativistic Heavy Ion Collider at $\sqrt{s_{NN}}=$ 7.7--62.4~GeV are presented for three centrality classes. The centrality dependence and the data at $\sqrt{s_{NN}}=$ 14.5~GeV are new. Except at the lowest beam energies we observe a similar relative $v_{2}$ baryon-meson splitting for all centrality classes which is in agreement within 15\% with the number-of-constituent quark scaling. The larger $v_{2}$ for most particles relative to antiparticles, already observed for minimum bias collisions, shows a clear centrality dependence, with the largest difference for the most central collisions. Also, the results are compared with A Multiphase Transport model and fit with a Blast Wave model