Azimuthal Charged-Particle Correlations and Possible Local Strong Parity Violation

Parity-odd domains, corresponding to nontrivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the system’s orbital momentum axis. We investigate a three-particle azimuthal correlator which is a P even observable, but directly sensitive to the charge separation effect. We report measurements of charged hadrons near center-of-mass rapidity with this observable in Au+Au and Cu+Cu collisions at √sNN=200  GeV using the STAR detector. A signal consistent with several expectations from the theory is detected. We discuss possible contributions from other effects that are not related to parity violation

Rapidity and species dependence of particle production at large transverse momentum for d+Au collisions at root S-NN=200 GeV

We determine rapidity asymmetry in the production of charged pions, protons, and antiprotons for large transverse momentum (p(T)) for d+Au collisions at root s(NN)=200 GeV. The rapidity asymmetry is defined as the ratio of particle yields at backward rapidity (Au beam direction) to those at forward rapidity (d beam direction). The identified hadrons are measured in the rapidity regions |y|\u3c 0.5 and 0.5 \u3c|y|\u3c 1.0 for the p(T) range 2.5 \u3c p(T)\u3c 10 GeV/c. We observe significant rapidity asymmetry for charged pion and proton+antiproton production in both the rapidity regions. The asymmetry is larger for 0.5 \u3c|y|\u3c 1.0 than for |y|\u3c 0.5 and is almost independent of particle type. The measurements are compared to various model predictions employing multiple scattering, energy loss, nuclear shadowing, saturation effects, and recombination and also to a phenomenological parton model. We find that asymmetries are sensitive to model parameters and show model preference. The rapidity dependence of pi(-)/pi(+) and (p) over bar /p ratios in peripheral d+Au and forward neutron-tagged events are used to study the contributions of valence quarks and gluons to particle production at high p(T)

Partonic flow and phi-meson production in Au+Au collisions at root s(NN)=200 GeV

We present first measurements of the phi-meson elliptic flow (v(2)(p(T))) and high-statistics p(T) distributions for different centralities from root s(NN) = 200 GeV Au+Au collisions at RHIC. In minimum bias collisions the v(2) of the phi meson is consistent with the trend observed for mesons. The ratio of the yields of the Omega to those of the phi as a function of transverse momentum is consistent with a model based on the recombination of thermal s quarks up to p(T)similar to 4 GeV/c, but disagrees at higher momenta. The nuclear modification factor (R-CP) of phi follows the trend observed in the K-S(0) mesons rather than in Lambda baryons, supporting baryon-meson scaling. These data are consistent with phi mesons in central Au+Au collisions being created via coalescence of thermalized s quarks and the formation of a hot and dense matter with partonic collectivity at RHIC

Forward Lambda production and nuclear stopping power in d+Au collisions at root s(NN)=200 GeV

We report the measurement of Lambda and (Lambda) over bar yields and inverse slope parameters in d+Au collisions at root s(NN)=200 GeV at forward and backward rapidities (y=+/- 2.75), using data from the STAR forward time projection chambers. The contributions of different processes to baryon transport and particle production are probed exploiting the inherent asymmetry of the d+Au system. Comparisons to model calculations show that baryon transport on the deuteron side is consistent with multiple collisions of the deuteron nucleons with gold participants. On the gold side, HIJING-based models without a hadronic rescattering phase do not describe the measured particle yields, while models that include target remnants or hadronic rescattering do. The multichain model can provide a good description of the net baryon density in d+Au collisions at energies currently available at the BNL Relativistic Heavy Ion Collider, and the derived parameters of the model agree with those from nuclear collisions at lower energies

Scaling properties of hyperon production in Au+Au collisions at root s(NN)=200 GeV

We present the scaling properties of Lambda, Xi, and Omega in midrapidity Au+Au collisions at the Brookhaven National Laboratory Relativistic Heavy Ion Collider at root s(NN)=200 GeV. The yield of multistrange baryons per participant nucleon increases from peripheral to central collisions more rapidly than that of Lambda, indicating an increase of the strange-quark density of the matter produced. The strange phase-space occupancy factor gamma(s) approaches unity for the most central collisions. Moreover, the nuclear modification factors of p, Lambda, and Xi are consistent with each other for 2 \u3c p(T) \u3c 5 GeV/c in agreement with a scenario of hadron formation from constituent quark degrees of freedom

Measurement of transverse single-spin asymmetries for dijet production in proton-proton collisions at root s=200 GeV

We report the first measurement of the opening angle distribution between pairs of jets produced in high-energy collisions of transversely polarized protons. The measurement probes (Sivers) correlations between the transverse spin orientation of a proton and the transverse momentum directions of its partons. With both beams polarized, the wide pseudorapidity (-1 \u3c=eta \u3c=+2) coverage for jets permits separation of Sivers functions for the valence and sea regions. The resulting asymmetries are all consistent with zero and considerably smaller than Sivers effects observed in semi-inclusive deep inelastic scattering. We discuss theoretical attempts to reconcile the new results with the sizable transverse spin effects seen in semi-inclusive deep inelastic scattering and forward hadron production in pp collisions

Transverse momentum and centrality dependence of High-p(T) nonphotonic electron suppression in Au plus Au collisions at root(NN)-N-S=200 GeV

The STAR collaboration at the BNL Relativistic Heavy-Ion Collider (RHIC) reports measurements of the inclusive yield of nonphotonic electrons, which arise dominantly from semileptonic decays of heavy flavor mesons, over a broad range of transverse momenta (1.2 \u3c p(T)\u3c 10 GeV/c) in p+p, d+Au, and Au+Au collisions at root s(NN)=200 GeV. The nonphotonic electron yield exhibits an unexpectedly large suppression in central Au+Au collisions at high p(T), suggesting substantial heavy-quark energy loss at RHIC. The centrality and p(T) dependences of the suppression provide constraints on theoretical models of suppression