The STAR Collaboration reports the first observation of exclusive rho(0) photoproduction, AuAu-->AuAurho(0), and rho(0) production accompanied by mutual nuclear Coulomb excitation, AuAu-->Au*Au*rho(0), in ultraperipheral heavy-ion collisions. The rho(0) have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at sqrt[s(NN)]=130 GeV agree with theoretical predictions treating rho(0) production and Coulomb excitation as independent processes.

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.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200A GeV S 1 S and 158A GeV Pb 1 Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions. The purpose of studying ultrarelativistic heavy-ion collisions is to understand the nature of hadronic matter under extreme conditions. Specifically, we are interested in a new form of matter, quark-gluon plasma, which may be produced in such collisions. Transverse momentum distributions are one of the most common tools used in studying high energy collisions. This is because the transverse motion is generated during the collision and hence is sensitive to the dynamics. More than 45 years ago, Fermi proposed a statistical method [1] to understand the results of high energy hadron-hadron collisions. Because of saturation of the phase space, the multiparticle production resulting from the high energy elementary collisions is consistent with a thermal description [1][2][3]. In heavy-ion collisions hydrodynamical behavior, that is, local thermal equilibrium and collective motion, may be expected because of the large number of secondary scatterings.It is now possible to identify and quantitatively measure the collective motion by systematic studies of results from different collision systems, using light (Si at BNL and S at CERN) and heavy (Au at BNL and Pb at CERN) ion beams [4][5][6]. A high degree of nuclear stopping and a strong Coulomb effect (also due to the high stopping) have already been reported in Pb 1 Pb central collisions [7,8]. In this Letter, we present transverse momentum distributions of pions, kaons, and protons, measured in the NA44 spectrometer, from Pb 1 Pb and S 1 S collisions. Results of calculations from a hydrodynamical model [5] will be used to aid in this analysis.The NA44 magnetic focusing spectrometer consists of two room-temperature dipoles and three superconducting quadruples. Particles originating from the target are focused at a plane about ten meters downstream and detected by a tracking system consisting of a pad chamberstrip chamber-scintillator hodoscope complex. Particle identification is done with two threshold Cherenkov counters and two highly segmented TOF hodoscopes. The phase-space coverage (transverse momentum p T vs rapidity y) is determined by the combination of the spectrometer angle (relative to the beam direction) and the nominal momentum setting of the magnets. The momentum resolution is typically s p ͞p # 0.2% and the TOF counters have an average time resolution of 100 ps. More details of the spectrometer can be found elsewhere [9].The spectrometer momentum range is 620% around the nominal values of 4 and 8 GeV͞c. For kaons and protons, the 8 GeV͞c setting was used and the rapidity coverage is (2.5-3.4) and (2.4-2.8) for kaons and protons, respectively. Two angular settings (44 and 130 mrad) were utilized in order ...

The PHENIX experiment at RHIC has measured transverse energy and charged particle multiplicity at mid-rapidity in Au + Au collisions at √ s N N = 19.6, 130 and 200 GeV as a function of centrality. The presented results are compared to measurements from other RHIC experiments, and experiments at lower energies. The √ s N N dependence of dET /dη and dN ch /dη per pair of participants is consistent with logarithmic scaling for the most central events. The centrality dependence of dET /dη and dN ch /dη is similar at all measured incident energies. At RHIC energies the ratio of transverse energy per charged particle was found independent of centrality and growing slowly with √ s N N . A survey of comparisons between the data and available theoretical models is also presented.

First results on charm quarkonia production in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) are presented. The yield of J/'s measured in the PHENIX experiment via electron-positron decay pairs at midrapidity for Au-Au reactions at ͱ s NN = 200 GeV is analyzed as a function of collision centrality. For this analysis we have studied 49.3ϫ 10 6 minimum bias Au-Au reactions. We present the J/ invariant yield dN/dy for peripheral and midcentral reactions. For the most central collisions where we observe no signal above background, we quote 90% confidence level upper limits. We compare these results with our J/ measurement from proton-proton reactions at the same energy. We find that our measurements are not consistent with models that predict strong enhancement relative to binary collision scaling.