System size dependence of freeze-out properties at RHIC Quark Matter 2006 Shanghai-China Nov System size dependence of freeze-out properties at RHIC STAR Aneta Iordanova, Olga Barannikova and Richard Hollis Motivation  Identified particle spectra in heavy-ion collisions at different center-of-mass energies and system size provide a unique tool to explore the QCD phase diagram.  Analysis of Cu+Cu data, collected by the STAR experiment, extends the systematic studies of bulk properties by addressing the energy and system size dependence of the freeze-out parameters at RHIC.  Bulk properties: - Kinetic freeze-out properties from spectral shapes T kin at kinetic freeze-out, transverse radial flow (β) - Chemical freeze-out properties from particle ratios T ch at chemical freeze-out, strangeness and baryon production Particle identification technique dE/dx distribution is normalized by the theoretical expectation for different particle types; e.g. for pion z(  )=ln[(dE/dx)/(dE/dx Th )]. z(particle) distribution is sliced into 0.25 MeV bins in p T at mid-rapidity, |y|<0.1, for 6 centrality bins for the top 60% of the cross-section. Consistent analysis technique is used for all low p T measurements for different center-of-mass energies and colliding systems. Raw yields are extracted for different particles from multi-Gaussian fits. PID spectra Particle spectra are measured for π ±, K ±, p and p-bar in Cu+Cu collisions at √s=200 and 62.4 GeV for 6 centrality bins. Mass dependence is observed in the slope of the spectra shapes. Kinetic freeze-out properties Blast-wave model The model assumes a boosted thermal source in transverse and longitudinal directions. 3 fit parameters are extracted: - flow velocity, β - kinetic freeze-out temperature, T kin - shape of the flow profile, n (PR C48 (1993) 2462) R r R Blast wave fits Blast wave fits are performed in each centrality bin simultaneously for π ±, K ±, p and p-bar. For the case of pions, the range below 0.5 GeV/c is excluded in order to reduce effects from resonance contributions. Kinetic freeze-out parameters All particle spectra for  ±, K ±, p and p-bar in Cu+Cu and Au+Au systems appear to be described by a common set of freeze-out parameters. These extracted freeze-out parameters are similar for an equivalent number of produced charged-particles, N ch - for both Cu+Cu and Au+Au systems - at both center-of-mass energies, 200 and 62.4 GeV. Cu+Cu 0-10%Au+Au 0-5% Kinetic Parameters 62.4 GeV200 GeV62.4 GeV200 GeV flow velocity  0.48± ± ± ±0.05 T kin (MeV)115±7112±993.8±4.189±12 Particle mean-p T * Bose-Einstein fits over the entire fiducial spectra range are used for the case of pions. Particle mean-p T increases with increasing centrality (increasing N ch ). This is consistent with an increase in radial flow with increasing centrality. Remarkable similarities as a function of number of produced charged-particles, N ch, are observed. -for both Cu+Cu and Au+Au systems - at both center-of-mass energies, 200 and 62.4 GeV. Testing hadro-chemistry Statistical model The model has 4 fit parameters: - chemical freeze-out temperature, T ch - baryon chemical potential, μ B - strangeness chemical potential, μ S - strangeness suppression, γ S Statistical model fits 10% central, Cu+Cu 200 GeV 10% central, Cu+Cu 62.4 GeV Chemical freeze-out parameters Universal chemical freeze-out temperature, T ch, is observed for all studied systems. Cu+Cu 0-10%Au+Au 0-5% Chemical Parameters62.4 GeV200 GeV62.4 GeV200 GeV T ch (MeV)152±8154±6156.2± ±5.8 μBμB 50±417±372.8± ±4.5 μSμS 4.5±2.82.6±2.4(0)11.1±2.63.9±2.6 γSγS 0.91± ± ± ±0.11 Summary  STAR has enlarged the variety of hadron spectra measurements at RHIC by providing new results for Cu+Cu at two different center-of-mass energies, √s=200 and 62.4 GeV.  The data has been studied within the framework of Kinetic and Chemical models to investigate the final hadronic state properties as a function of energy and centrality.  This multi-dimensional systematic study has revealed the importance of the initial collision geometric overlap. T kin (GeV) Minimum Bias, Cu+Cu 200 GeV0-10% central, Cu+Cu 200 GeV 0.50<p T <0.55 GeV/c STAR Preliminary 10% central, 200 GeV STAR Preliminary 10% central, 62.4 GeV 10% central, 200 GeV10% central, 62.4 GeV Preliminary values of freeze-out parameters Are the bulk properties entropy driven? Similar systematic dependence is observed for different ion species as a function of centrality. Preliminary values of freeze-out parameters STAR Preliminary 5% central10% centralMin-Bias for the STAR collaboration