Identified pi(+/-), K(+/-), p, and (-)p transverse momentum spectra at midrapidity in sqrt[s(NN)] = 130 GeV Au+Au collisions were measured by the PHENIX experiment at RHIC as a function of collision centrality. Average transverse momenta increase with the number of participating nucleons in a similar way for all particle species. Within errors, all midrapidity particle yields per participant are found to be increasing with the number of participating nucleons. There is an indication that K(+/-), p, and (-)p yields per participant increase faster than the pi(+/-) yields. In central collisions at high transverse momenta (p(T) > or =2 GeV/c), (-)p and p yields are comparable to the pi(+/-) yields.

Measurements of the ratio of Drell-Yan yields from an 800 GeV/c proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360,000 Drell-Yan muon pairs remained after all cuts on the data. ¿From these data, the ratio of anti-down (d) to anti-up (ū) quark distributions in the proton sea is determined over a wide range in Bjorken-x. These results confirm previous measurements by E866 and extend them to lower x. From these data, (d −ū) and (d −ū)dx are evaluated for 0.015 < x < 0.35. These results are compared with parameterizations of various parton distribution functions, models and experimental results from NA51, NMC and HERMES. 13.85.Qk; 14.20.Dh; 24.85.+p; 14.65.Bt

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.

A precise measurement of the ratios of the Drell-Yan cross section per nucleon for an 800 GeV/c proton beam incident on Be, Fe and W targets is reported. The behavior of the Drell-Yan ratios at small target parton momentum fraction is well described by an existing fit to the shadowing observed in deep-inelastic scattering. The cross section ratios as a function of the incident parton momentum fraction set tight limits on the energy loss of quarks passing through a cold nucleus. The Drell-Yan process, where a beam quark (antiquark) fuses with a target antiquark (quark) producing a muon pair, can be used to study the interactions of fast partons penetrating through cold nuclei. Only initial state interactions are important in Drell-Yan since the dimuon in the final state does not interact strongly with the partons in the medium. This makes Drell-Yan scattering an ideal tool to study energy loss of fast quarks in nuclear matter by comparing the observed yields from a range of nuclear targets. The dynamics of fast parton energy loss in nuclear matter is the subject of considerable theoretical interest [1][2][3][4] and has significant implications for the physics of relativisitic heavy ion collisions.Drell-Yan scattering is closely related to deep-inelastic scattering (DIS) of leptons, but unlike DIS it can be used specifically to probe antiquark contributions in target parton distributions. When DIS on nuclei occurs at x < 0.08, where x is the parton momentum fraction, the cross section per nucleon decreases with increasing nuclear number A due to shadowing [5,6].