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Haibin Zhang / Brookhaven National Laboratory For the STAR Collaboration

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

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Haibin Zhang / Brookhaven National Laboratory For the STAR Collaboration

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

  2. Kinetic Freeze-Out t? Chemical Freeze-Out Colliding Relativistic Heavy Ion Collisions Motivation - II Resonances!!! Haibin Zhang 2

  3. Resonances’ 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 Can be heavy mesons  different physics compared to light stable mesons Resonances measured in STAR ResonanceK*(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 Resonance in Heavy Ion Collisions - I Motivation - II Resonances are strongly decaying particles which have lifetimes a few fm/c 1 fm/c ~ 10–23 second Haibin Zhang 3

  4. 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 4

  5.  +  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 5

  6. Resonance Signal K*0K+- 0 +-  K+K- STAR Preliminary *± ± * pK- ++ p+ Haibin Zhang 6

  7. Resonance Spectra 0 in Au+Au K*0  0 in pp f0in Au+Au STAR Preliminary ++ f0in pp (1520) Haibin Zhang 7

  8. 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 8

  9. Thermal Models - I 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 9

  10. Δ++ p ρ0 π- K*0 K- Thermal Models - II Chemical freeze-out Au+Au Kinetic freeze-out Chemical = Kinetic freeze-out sNN = 200 GeV mρ=700MeV mρ=770MeV Statistical and systematic errors added in quadrature Haibin Zhang 10

  11. Λ(1520) Λ f0 π- Φ K- Thermal Models - III Chemical freeze-out sNN = 200 GeV Chemical freeze-out = Kinetic freeze-out Au+Au • Thermal calculations  Do not reproduce ratios of short-lived resonances Statistical and systematic errors added in quadrature Haibin Zhang 11

  12. 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 12

  13. RHIC Initial Production Final Production UrQMD Transport ModelUrQMD dN/dt SPS <E>/<N> M. Bleicher QM 2002 t (fm/c) Haibin Zhang 13

  14. t -  N(t) = N0 e t K*0 K*0 K- K- 0.205 Au+Au = Δt - 0.385  p+p e = Time Scale - I 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 14

  15. Life time: K(892) = 4 fm/c L(1520) = 13 fm/c Time Scale - II G. Torrieri and J. Rafelski, Phys. Lett. B509 (2001) 239 • Model includes: • Temperature at chemical freeze-out • “Life-time” between chemical and • thermal freeze-out • By comparing two particle ratios • (no regeneration) • results between : • T= 160 MeV =>  > 4 fm/c • (lower limit !!!) •  = 0 fm/c => T= 110-130 MeV Does not work for S*, D++, f (STAR) (1520)/ = 0.034  0.011  0.013 K*/K- = 0.20  0.03 at 0-10% most central Au+Au Haibin Zhang 15

  16. K l+l- K  K+K-  c=50fm AMPT, STAR Nucl-th/0202086  Vector Meson - I 130GeV • vector meson sensitive to early medium effect • AMPT: (l+l-)/(K+K-)=1.5 • Experiments’ comparison Haibin Zhang 16

  17. PHENIX Preliminary PHENIX Preliminary fKK fee  Vector Meson - II 200GeV K+K- Sensitivity Reach e+e- J. Nagle, QM02 Haibin Zhang 17

  18. Jet Quenching The high pT suppression behavior is different for KS0 and  Is this a mass effect or depending on particle species (meson vs. baryon) ? Hydrodynamic Model vs. Quark Recombination Model K* and : mesons but their masses Close to  K* and RAA/RCP can distinguish this difference Haibin Zhang 18

  19. Jet Quenching STAR Preliminary Year-II 200 GeV K* RAA/RCP close to KS0 Year-IV 200 GeV  RCP(0-5%/40-60%) close to KS0 Haibin Zhang 19

  20. Elliptic Flow v2 Elliptic flow(v2) carry information of initial stage Hydrodynamic model can describe data well at low pt while the intermediate pt range is described by coalescence model Phys. Rev. Lett. 92 (2004) 052302 Haibin Zhang 20

  21. Elliptic Flow v2 0-80% STAR Preliminary Year-II 200 GeV K* v2needs more statistics  v2 close to KS0 with ~1 effect Year-IV 200 GeV Haibin Zhang 21

  22. 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  vector meson  and K* nuclear modification factor RAA/RCP and elliptic flow v2 close to KS0  prefer the particle species (baryon/meson) effect Thanks! Haibin Zhang 22

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