1. 24 June 2007 Strangeness in Quark Matter 2007 STAR 2S0Q0M72S0Q0M7 Strangeness and bulk freeze- out properties at RHIC Aneta Iordanova

2. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Outline Motivation STAR Experiment Analysis Techniques Results Summary

3. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 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. 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

4. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 STAR Experiment Time Projection Chamber Measures charged particle momenta and energy loss within |  |<1 Full azimuthal acceptance

5. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Analysis Technique Exploit the ionization energy loss (dE/dx) Distribution is normalized by the theoretical expectation for different particle types; e.g. z(  )=ln[(dE/dx)/(dE/dxTh)]. z(  ) distribution is sliced into  p=50MeV/c for |y|<0.1 (mid-rapidity) 6 centrality bins - corresponding to the top 60% of the cross-section. √s NN =62.4GeV and 200 GeV 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. Cu+Cu 200 GeV 0-10% central 0.50<p T <0.55 GeV/c Cu+Cu 200 GeV Minimum Bias pion kaon electron proton normalized STAR Preliminary

6. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Results – Particle Spectra Raw data are corrected for: Single particle (track finding) efficiency Energy Loss in the TPC Pions/protons corrected for background Corrected spectra are shown for m T -m 0 Central 0-10% data Mass dependence is observed in the slope of the spectra shapes STAR Preliminary 10% central, Cu+Cu 200 GeV 62.4 GeV –– –– p

7. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Results – Particle Ratios K – /  – ratio Systematically higher in the smaller system p/  – ratio Same for Au+Au and Cu+Cu STAR Preliminary

8. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Fitting the data – 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 STAR Preliminary 10% central, Cu+Cu @ 62.4 GeV10% central, Cu+Cu @ 200 GeV Fit uses only data from , K and p

9. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Chemical freeze-out properties Strangeness suppression factor consistent within errors with Au+Au results for the central data But appears to be systematically higher for the smaller system STAR Preliminary

10. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Chemical freeze-out properties Chemical freeze-out temperature Universal chemical freeze-out temperature, T ch, is observed for all studied systems. 5% central10% centralMin-Bias STAR Preliminary

11. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Chemical freeze-out properties Chemical freeze-out temperature Similar systematic dependence is observed for different ion species as a function of centrality. STAR Preliminary

12. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Fitting the data – 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

13. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Fit Results 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. 10% central, Cu+Cu @ 62.4 GeV10% central, Cu+Cu @ 200 GeV STAR Preliminary

14. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Particle mean-p T Particle mean-p T : increases with centrality (N ch ). consistent with an increase in radial flow with centrality. Remarkable similarities as a function of N ch are observed. for both Cu+Cu and Au+Au systems at both center-of-mass energies, 200 and 62.4 GeV. Are the bulk properties entropy driven? * Bose-Einstein fits over the entire fiducial spectra range are used for the case of pions.

15. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 STAR Preliminary Kinetic freeze-out properties All particle spectra for p ±, K ±, p and p-bar in Cu+Cu and Au+Au systems appear to be described by a common set of freeze- out parameters. Cu+Cu 0-10%Au+Au 0-5% Kinetic Parameters62.4 GeV200 GeV62.4 GeV200 GeV flow velocity  0.48±0.060.52±0.070.54±0.010.59±0.05 T kin (MeV)115±7112±993.8±4.189±12

16. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 Kinetic freeze-out properties STAR Preliminary These extracted freeze-out parameters are found to be similar for an equivalent N ch for both Cu+Cu and Au+Au systems at both center-of-mass energies, 200 and 62.4 GeV.

17. STAR 2S0Q0M72S0Q0M7 24 June 2007Aneta IordanovaStrangeness in Quark Matter 2007 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. Thermal models used for studying kinetic and chemical freeze- out properties point to a systematic dependence on the number of charged particles produced Independent of collision system and energy This multi-dimensional systematic study has revealed the importance of the initial collision geometric overlap.