1. 1 相对论重离子碰撞中  介子的产生 陈金辉 中国科学院上海应用物理研究所 QCD 相变与重离子碰撞物理国际暨 第七届全国研讨会 @USTC 2008 年 7 月 10 号 -12 号 Many thanks to: X. Cai, S. Blyth, F. Jin, H. Huang, G. Ma, J. Ma, B. Mohanty, N. Xu …

2. 2 Outline Introduction – What we have learnt from RHIC Motivation – Why we focus on  -meson? Results –  -meson elliptic flow measurement –  -meson spectra measurement –  /  and  /  ratio Conclusion and Outlook

3. 3 p T Scales and Physical Processes R CP Three P T Regions: -- Fragmentation (high p T jet energy loss) -- multi-parton dynamics (recombination or coalescence or …) -- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? )

4. 4 High p T suppression Very dense matter has been created in central Au+Au collisions The dense matter is responsible for the suppression of high p T particles and the disappearance of back-to-back correlation

5. 5 The Suppression is the Same for   and  – Parton level effect No suppression  photons don’t participate!

6. 6 High p T phenomena at RHIC Very dense matter has been created in central Au+Au collisions! This dense matter is responsible for the suppression of high p T particles and the disappearance of back-to-back correlation! The energy loss observed at RHIC is in parton level, but the mechanism for parton energy loss is yet to be understood! [Won’t elaborate in this talk.]

7. 7 Intermediate p T, large p/  ratio Unexpected large p/  ratio in central Au+Au collisions – The hadronization scheme should be different from e + e - !

8. 8 Intermediate p T, v 2 and R CP grouping STAR PHENIX Baryon Meson V 2 and R CP for PID measurement shown a B/M grouping behavior – partonic degree of freedom?

9. 9 What can we learn from those phenomena? At RHIC intriguing experimental features: – enhanced baryon over meson production – strong elliptic flow – grouping behavior of v 2 and R CP for PID ? Hadronization of bulk dense matter created at RHIC should be different from e + e - collisions! ? Quark Coalescence/Recombination ? Evidence for Deconfinement ? Possible for mass-effect rather than B/M type  are particularly important probes for these issues!

10. 10 Why  -meson ? [1] A. Shor, Phys. Rev. Lett. 54 (1985) 1122 K+K+ K-K- K-K- K+K+ φ φ φ K+K+ K-K- QGP The  -meson is a clean probe from early time: ● Small  for interactions with non-strange particles [1] ● Relatively long-lived (41 fm/c) → decays outside the fireball The  -meson  can provide info on particle production mechanisms /medium constituents: ● The  is a meson but as heavy as , p baryons (mass vs. particle type?) An interesting probe to understand the strangeness dynamics: ● No net strangeness in the initial colliding nuclei

11. 11  -meson measurement, v 2 and R CP For v 2 and R CP measurement,  -meson follows the trend observed in the K s,  mesons rather than in the  p baryons – clear signature for the Coa./Reco. hadronization mechanism. STAR Col. Phys. Rev. Lett. 99, (2007) 112301.

12. 12  -meson production at RHIC  /K - STAR Col. Phys. Lett. B 612, (2005) 181, Phys. Rev. Lett. 99, (2007) 112301. 1. Evolution in the centrality dependence; 2.,  -meson may decouple early; 3. N(  )/N(K), ruled out the K-coalescence.

13. 13  ’s  are mostly from bulk s quarks N(  )/N(  ) vs. p T is consistent with a model based on the recombination of thermal s quarks up to p T ~ 4.0 GeV/c, but disagrees at higher p T. v 2 (  ) shows similar behavior as PID’s, positive signature for partonic collectivity at RHIC. STAR Col. Phys. Rev. Lett. 99, (2007) 112301.

14. 14 Parton p T distributions at Hadronization? Can we extract the strange quark p T distribution from multi-strange hadron data? If baryons of p T are mostly formed from coalescence of partons at p T /3 and mesons of p T are mostly formed from coalescence of partons at p T /2  and  particles have no decay feed-down contribution! These particles will freeze-out earlier from the system and have small hadronic rescattering cross sections [1,2]. [1] A. Shor, PRL 54 (1985) 1122; [2] H. Van Hecke et al., PRL 81 (1998) 5764.

15. 15 Strange and down quark distribution Strange quark distributions are flatter than light quarks! arXiv:0801.2265

16. 16 Test on s/d ratio at hadronization s/d quark ratios =  =  Yes! but with large uncertainties due to decay feed-down corrections in  arXiv:0801.2265 X. Wang

17. 17 Summary and Conclusion N(  )/N(K) vs. N part rules out the Kaon coalescence as a dominant channel for  production at RHIC; N(  )/N(  ) vs. p T favors the model prediction that  s are made via thermalied s-quarks coalescence at RHIC; v 2 (  ) vs. p T conclude that the partonic collectivity has been formed at RHIC; N(  )/N(  ) and N(  )/N(  ) vs. p T /n q indicate that strange quarks may have developed a stronger collective radial flow than the light quarks during the initial parton evolution at RHIC; Since  mesons are made via coalescence of seemingly thermalized s quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC.

18. 18 Outlook: Extend PID Capability ToF detector updated: — 5 trays of ToF system installed in Run 8, commissioned, and used for physics. — 90 (of 120) ToF trays to be installed for Run 9 and will be completed before Run 10.  /K separation to 1.6 GeV/c (0.65 TPC) (  +K)/p to 3 GeV/c (1.1 TPC) Clean electron ID down to 0.2 GeV

19. 19 The location of the QCD Critical Point Outlook: RHIC is ready for the Beam Energy Scan Hadron gas QGP sketch by P. Sorensen Key measurements — PID spectra and v 2 — K/ , … fluctuation