相对论重离子碰撞中 f 介子的产生

1. 相对论重离子碰撞中f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. Introduction What we have learnt from RHIC Motivation Why we focus on f-meson? Results f-meson elliptic flow measurement f-meson spectra measurement W/f and X/f ratio Conclusion and Outlook Outline

3. pT Scales and Physical Processes RCP Three PT Regions: -- Fragmentation (high pT jet energy loss) -- multi-parton dynamics (recombination or coalescence or …) -- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? )

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

5. The Suppression is the Same for p0 and h – Parton level effect No suppression  photons don’t participate!

6. High pT phenomena at RHIC • Very dense matter has been created • in central Au+Au collisions! • This dense matter is responsible for the suppression of high pT particlesandthe • 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. Intermediate pT, large p/p ratio • Unexpected large p/p ratio in central Au+Au collisions • The hadronization scheme should be different from e+e- !

8. STAR Baryon Meson PHENIX Intermediate pT, v2 and RCPgrouping • V2 and RCP for PID measurement shown a B/M grouping behavior • partonic degree of freedom?

9. What can we learn from those phenomena? • At RHIC intriguing experimental features: • enhanced baryon over meson production • strong elliptic flow • grouping behavior of v2 and RCP 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 f are particularly important probes for these issues!

10. K- K+ K+ • The f-mesoncan provide info onparticle productionmechanisms/medium constituents: K- • The f is a meson but as heavy asL,p baryons (mass vs. particle type?) φ • Thef-meson is aclean probefrom early time: φ QGP • Smallsfor interactions with non-strange particles[1] • Relatively long-lived (41 fm/c) →decays outside the fireball φ • An interesting probe to understand thestrangeness dynamics: K- • No net strangeness in the initial colliding nuclei K+ Why f-meson ? [1] A. Shor, Phys. Rev. Lett. 54 (1985) 1122

11. f-meson measurement, v2 and RCP • For v2 and RCP measurement, f-meson follows the trend observed in the Ks,p mesons rather than in the L, pbaryons • clear signature for the Coa./Reco. hadronization mechanism. STAR Col. Phys. Rev. Lett. 99, (2007) 112301.

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

13. N(W)/N(f) vs. pT is consistent with a model based on the recombination of thermal s quarks up to pT ~ 4.0 GeV/c, but disagrees at higher pT. • v2(f) shows similar behavior as PID’s, positive signature for partonic collectivity at RHIC. STAR Col. Phys. Rev. Lett. 99, (2007) 112301. f’sare mostly from bulk s quarks

14. Parton pT distributions at Hadronization? Can we extract the strange quark pT distribution from multi-strange hadron data? If baryons of pT are mostly formed from coalescence of partons at pT/3 and mesons of pT are mostly formed from coalescence of partons at pT/2 • and f 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. Strange and down quark distribution arXiv:0801.2265 Strange quark distributions are flatter than light quarks!

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

17. 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. Summary and Conclusion • N(f)/N(K) vs. Npart rules out the Kaon coalescence as a dominant channel for f production at RHIC; • N(W)/N(f) vs. pT favors the model prediction that fs are made via thermalied s-quarks coalescence at RHIC; • v2(f) vs. pT conclude that the partonic collectivity has been formed at RHIC; • N(W)/N(f) and N(X)/N(f) vs. pT/nq indicate that strange quarks may have developed a stronger collective radial flow than the light quarks during the initial parton evolution at RHIC;

18. Outlook: Extend PID Capability • /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 • 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.

19. sketch by P. Sorensen QGP Hadron gas Outlook: RHIC is ready for the Beam Energy Scan • Key measurements • PID spectra and v2 • K/p , <pT> … fluctuation The location of the QCD Critical Point