Measurement of the Parity-Violating Longitudinal Single-Spin Asymmetry for W-+/- Boson Production in Polarized Proton-Proton Collisions at root s=500 GeV

We report the first measurement of the parity-violating single-spin asymmetries for midrapidity decay positrons and electrons from W+ and W- boson production in longitudinally polarized proton-proton collisions at root s = 500 GeV by the STAR experiment at RHIC. The measured asymmetries, A(L)(W+) = -0.27 +/- 0.10(stat.) +/- 0.02(syst.) +/- 0.03(norm.) and A(L)(W-) = 0.14 +/- 0.19(stat.) +/- 0.02(syst.) +/- 0.01(norm.), are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized deep-inelastic scattering measurements

Beam-Energy Dependence of Charge Separation along the Magnetic Field in Au plus Au Collisions at RHIC

Local parity-odd domains are theorized to form inside a quark-gluon plasma which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect. The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this Letter, we present the results of the beam-energy dependence of the charge correlations in Au + Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39, and 62.4 GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy and tends to vanish by 7.7 GeV. This implies the dominance of hadronic interactions over partonic ones at lower collision energies

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

Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at sNN =200 GeV

The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at sNN=200 GeV. Charged jets are reconstructed with the anti-kT algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents (pT>0.2 GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at midrapidity, for charged-jet transverse momentum pT,jetch<30 GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at s=2.76 TeV, to calculations for p+p collisions at s=200 GeV based on the pythia Monte Carlo generator and on a next-to-leading order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of 2.8\ub10.2(stat)\ub11.5(sys) GeV/c, for 10<20 GeV/c and R=0.5. No medium-induced change in jet shape is observed for R<0.5. The azimuthal distribution of low-pT,jetch recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasiparticles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au+Au collisions of 50\ub130(sys)% of the large-angle yield in p+p collisions predicted by pythia

Centrality and Transverse Momentum Dependence of Elliptic Flow of Multistrange Hadrons and phi Meson in Au plus Au Collisions at root S NN=200 GeV

We present high precision measurements of elliptic flow near midrapidity (vertical bar y vertical bar < 1.0) for multistrange hadrons and phi meson as a function of centrality and transverse momentum in Au + Au collisions at center of mass energy root S (NN) = 200 GeV. We observe that the transverse momentum dependence of phi and Omega v(2) is similar to that of pi and p, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0%-30% and 30%-80% collision centrality. There is an indication of the breakdown of previously observed mass ordering between phi and proton v(2) at low transverse momentum in the 0%-30% centrality range, possibly indicating late hadronic interactions affecting the proton v(2)