Systematics of identified particle production in proton-proton, deuteron-gold and gold+gold collisions at RHIC energies
Abstract
Identified mid-rapidity particle spectra and freeze-out properties are presented for 200 GeV pp, 200 GeV dAu and 62.4 GeV Au-Au collisions, measured in the STAR-TPC. The STAR-TPC is a unique tool to investigate identified bulk particle production from elementary pp to large multiplicity Au-Au collisions. Results are contrasted to previous experiments to provide an overview of bulk properties in heavy-ion collisions. Evolution of the identified particle spectra (π±, K±, p and p¯) with charged particle multiplicity and event centrality is investigated in detail. Significant hardening of the spectrum of heavy particles (kaons and protons/antiprotons) is found in central Au-Au collisions. The average transverse momentum of kaons and protons/antiprotons in high multiplicity pp and central dAu collisions is larger than in peripheral Au-Au collisions at the same energy. The average transverse momentum in 62.4 GeV and 200 GeV Au-Au collisions seem to only depend on event multiplicity. Particle production examined through particle-antiparticle ratios (π +/π−, K+/ K−, p¯/p) and unlike particle ratios (K−/π−, p¯/π−) show smooth evolution from pp to dAu to Au-Au collisions. Significant net baryon is present in the central collision zone in 62.4 GeV collisions and 200 GeV collisions. Strangeness production increases with centrality in peripheral collisions and saturates in medium-central to central collisions in heavy-ion collisions at 62.4 and 200 GeV, in contrast to lower SPS and AGS energies. Chemical freeze-out properties of the collision systems are obtained from particle ratios and the kinetic freeze-out properties from the shapes of particle spectra. Thermal model fits to the measured particle ratios yield a chemical freeze-out temperature ∼ 155 MeV in 200 GeV pp; 200 GeV dAu and 62.4 GeV Au-Au collisions. The extracted chemical freeze-out temperature is close to the critical phase transition temperature predicted by lattice QCD calculations. The kinetic freeze-out temperature extracted from hydrodynamically motivated blast-wave models shows a continuous drop from pp, dAu and peripheral to central Au-Au collisions, while the transverse flow velocity increases from ∼ 0.2 in pp to ∼ 0.6 in central 200 GeV Au-Au collisions. The kinetic freeze-out parameters in 62.4 GeV and 200 GeV Au-Au collisions seem to be governed only by event multiplicity/centrality. The kinetic freeze-out results are obtained from blast-wave fit to spectra data treating all particles as primordial ones. However, resonance decays may modify the spectral shapes significantly, and therefore may affect the extrapolated kinetic freeze-out parameters. In order to study this possible effect the data are fitted with the blast-wave model including resonances. It is found that the thus extracted parameters are consistent with those obtained without including resonances. This is because the resonance decays do not modify the spectral shapes significantly in the measured pT, region in STAR.
Degree
Ph.D.
Advisors
Wang, Purdue University.
Subject Area
Nuclear physics
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