Haibin Zhang / Brookhaven National Laboratory For the STAR Collaboration

1. Resonance Production at STAR Physics Motivations  Resonance in Heavy-Ion Collisions Measurement Technique Haibin Zhang / Brookhaven National Laboratory For the STAR Collaboration Results and Discussions Resonance Signal and Spectra Particle Ratios Thermal Model Comparison Time Scale Summary Haibin Zhang 1

2. Kinetic Freeze-Out Resonances!!! Chemical Freeze-Out t? Colliding Unique properties compared to stable particles: Short lifetime  directly measure particle properties in medium, such as mass Decayed daughters can undergo a period of re-interaction in the hadron gas Resonance in Heavy Ion Collisions - I Resonances are strongly decaying particles which have lifetimes a few fm/c Haibin Zhang 2

3. π K* K K* measured π K* K*lost K π K* π π K* K K K K* measured Resonance in Heavy Ion Collisions - II Motivation - II • If resonance decays before kinetic freeze-out  not reconstructed due to rescattering of daughters • K*0(c = 4 fm)survival probability timebetween chemical and kinetic freeze-out, source size and pT of K*0 • Chemical freeze-out elastic interactionsπK  K*0πKregenerate K*0(892)until kinetic freeze-out • K*0/K may reveal time between chemical and kinetic freeze-out • Λ*/Λ,Δ++/p,ρ0/π, f0/π? Chemical freeze-out Kinetic freeze-out time Haibin Zhang 3

4.  +  0  K+ K*0 *+ p  + K* Measurement Techniques • Event-Mixing technique (for example: K*0K+) • Select K+ and tracks from PID by energy loss in TPC • Combine all pairs from same event  Signal+Background (same event spectra) • Combine pairs from different events Background (mixed event spectra) • Signal = same event spectra – mixed event spectra • Like-Sign technique (for example: 0+) • Combine +  – pairs  same event spectra • Combine + + pairs and  –  – pairs  • background spectra • Signal = +  – – 2   + +   –  – • Features • Enable us to measure very short lived resonances • with high efficiency (~ 80%) • Reconstruction is not done particle by particle • Need lots of statistics in order to overcome large • combinatorial background Haibin Zhang 4

5. Resonance Signal K*0K+- 0 +-  K+K- STAR Preliminary *± ± * pK- ++ p+ Resonance K*(892) (770) f0(980) (1020) (1232) (1520) (1385) Decay channel K  K K p  pK   Branching Ratio % ~100 ~100 dominant 49.2 >99 22.5 88.2 Width [MeV] 50.7 150 40 to 100 4.46 ~120 15.6 35.8 Life time [fm/c] 4 1.3 40 ~1.6 13 5.6 Haibin Zhang 5

6. Resonance Spectra 0 in Au+Au K*0  0 in pp f0in Au+Au STAR Preliminary ++ f0in pp (1520) Haibin Zhang 6

7. Particle Ratios Statistical and systematic errors added in quadrature • K*/Kand*/in Au+Au significantly smaller than in p+p • /pand*/in Au+Au larger than in p+p • Φ/K-independent of centrality in Au+Au and close to p+p • /andf0/in peripheral Au+Au close to p+p Haibin Zhang 7

8. Thermal Models PLB 518 (2001) 41 Grand canonical ensemble Quantum conservation laws Markers: measured data Lines: model predictions STAR Preliminary Stable particle ratios can be successfully predicted by thermal model Strangeness enhancement observed in Au+Au Discrepancies exist for resonance ratios between measured data points and model predictions Haibin Zhang 8

9. Hadronic Interactions M. Bleicher et al. J. Phys. G 25 (1999) 1859 () determines the rescattering effect for K*, ,  Large lifetime of  (40 fm/c)  weak rescattering σ(Kπ) (K), (), (p), (KK)determine the regeneration effect for K*, ,  and , respectively (p) > () >> (K) >> (KK)  K*/K suppression, /p enhancement, flat / and /K σ(ππ) Indication of hadronic interactions between freeze-outs Tch freeze-out Tch from Thermal model, Tkin from Blast-Wave-Fit to p, K and p σ(πp) Tkin freeze-out PRL 92 (2004) 112301 Haibin Zhang 9

10. t -  N(t) = N0 e t K*0 K*0 K- K- 0.205 Au+Au = Δt - 0.385  p+p e = Time Scale If we only consider the rescattering process The time between chemical and kinetic freeze-out should be Δt ~ 3 fm/c If regeneration process is included, Δt > 3 fm/c Haibin Zhang 10

11. Summary K*, , f0, , , * and * resonances measured in Au+Au and p+p collisions Resonance rescattering and regeneration effect in hadronic phase Discrepancy between thermal model predictions and measured data Indications of late stage hadronic interactions. Resonance can be used as a clock to measure the time scale between chemical and thermal freeze-outs Thanks! Haibin Zhang 11