No abstract

The centrality dependence of transverse momentum distributions and yields for ± , K ± , p, and p in Au + Au collisions at ͱ s NN = 200 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. We observe a clear particle mass dependence of the shapes of transverse momentum spectra in central collisions below ϳ2 GeV/ c in p T. Both mean transverse momenta and particle yields per participant pair increase from peripheral to midcentral and saturate at the most central collisions for all particle species. We also measure particle ratios of − / + , K − / K + , p / p, K / , p / , and p / as a function of p T and collision centrality. The ratios of equal mass particle yields are independent of p T and centrality within the experimental uncertainties. In central collisions at intermediate transverse momenta ϳ1.5-4.5 GeV/ c, proton and antiproton yields constitute a significant fraction of the charged hadron production and show a scaling behavior different from that of pions.

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured electrons with 0:3 < p T < 9 GeV=c at midrapidity (jyj < 0:35) from heavy-flavor (charm and bottom) decays in Au Au collisions at s NN p 200 GeV. The nuclear modification factor R AA relative to p p collisions shows a strong suppression in central Au Au collisions, indicating substantial energy loss of heavy quarks in the medium produced at RHIC energies. A large azimuthal anisotropy v 2 with respect to the reaction plane is observed for 0:5 < p T < 5 GeV=c indicating substantial heavy-flavor elliptic flow. Both R AA and v 2 show a p T dependence different from those of neutral pions. A comparison to transport models which simultaneously describe R AA p T and v 2 p T suggests that the viscosity to entropy density ratio is close to the conjectured quantum lower bound, i.e., near a perfect fluid.