Strangeness and bulk freeze-out properties at RHIC

1. Strangeness and bulk freeze-out properties at RHIC Aneta Iordanova

2. Outline • Motivation • STAR Experiment • Analysis Techniques • Results • Summary Aneta Iordanova

3. 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 • Tkin at kinetic freeze-out, transverse radial flow (β) • - Chemical freeze-out properties from particle ratios • Tch at chemical freeze-out, strangeness and baryon production Aneta Iordanova

4. STAR Experiment • Time Projection Chamber • Measures charged particle momenta and energy loss within |h|<1 • Full azimuthal acceptance Aneta Iordanova

5. Analysis Technique normalized • Exploit the ionization energy loss (dE/dx) • Distribution is normalized by the theoretical expectation for different particle types; e.g. z(p)=ln[(dE/dx)/(dE/dxTh)]. • z(p) distribution is sliced into dp=50MeV/c for • |y|<0.1 (mid-rapidity) • 6 centrality bins - corresponding to the top 60% of the cross-section. • √sNN=62.4GeV and 200 GeV • Consistent analysis technique is used for all low pT measurements for different center-of-mass energies and colliding systems. • Raw yields are extracted for different particles from multi-Gaussian fits. pion kaon electron proton Cu+Cu 200 GeV Minimum Bias Cu+Cu 200 GeV 0-10% central 0.50<pT<0.55 GeV/c STAR Preliminary Aneta Iordanova

6. p– K– p Results – Particle Spectra 10% central, Cu+Cu • 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 mT-m0 • Central 0-10% data • Mass dependence is observed in the slope of the spectra shapes 200 GeV 62.4 GeV STAR Preliminary Aneta Iordanova

7. Results – Particle Ratios • K–/p– ratio • Systematically higher in the smaller system • p/p– ratio • Same for Au+Au and Cu+Cu STAR Preliminary Aneta Iordanova

8. Fitting the data – Statistical Model • The model has 4 fit parameters: • chemical freeze-out temperature, Tch • baryon chemical potential, μB • strangeness chemical potential, μS • strangeness suppression, γS STAR Preliminary 10% central, Cu+Cu @ 62.4 GeV 10% central, Cu+Cu @ 200 GeV Fit uses only data from p, K and p Aneta Iordanova

9. STAR Preliminary 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 Aneta Iordanova

10. STAR Preliminary Min-Bias 10% central 5% central Chemical freeze-out properties • Chemical freeze-out temperature • Universal chemical freeze-out temperature, Tch, is observed for all studied systems. Aneta Iordanova

11. STAR Preliminary Chemical freeze-out properties • Chemical freeze-out temperature • Similar systematic dependence is observed for different ion species as a function of centrality. Aneta Iordanova

12. The model assumes a boosted thermal source in transverse and longitudinal directions. 3 fit parameters are extracted: flow velocity, β kinetic freeze-out temperature, Tkin shape of the flow profile, n R r R Fitting the data – Blast –wave model (PR C48 (1993) 2462) Aneta Iordanova

13. 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. Fit Results STAR Preliminary 10% central, Cu+Cu @ 62.4 GeV 10% central, Cu+Cu @ 200 GeV Aneta Iordanova

14. Particle mean-pT • Particle mean-pT: • increases with centrality (Nch). • consistent with an increase in radial flow with centrality. • Remarkable similarities as a function of Nch 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. Aneta Iordanova

15. 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. STAR Preliminary Kinetic freeze-out properties Aneta Iordanova

16. STAR Preliminary STAR Preliminary Kinetic freeze-out properties • These extracted freeze-out parameters are found to be similar for an equivalent Nch • for both Cu+Cu and Au+Au systems • at both center-of-mass energies, 200 and 62.4 GeV. Aneta Iordanova

17. 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. Aneta Iordanova