Strange Probes of QCD Matter Huan Zhong Huang Department of Physics and Astronomy University of California Los Angeles, CA Oct 6-10, 2008; SQM2008 Beijing Thanks to Jinhui Chen, Gang Wang and Shingo Sakai

Outline 1) Strangeness in Bulk Partonic Matter 2) Hadronization and Evolution Dynamics 3) Thermalized Effective Quarks 4) P T Scale for Jet Energy Loss in QCD Medium 5) Is There a Clear Path-Length Effect in Eloss? 6) Ourlook 2

3 Strangeness Probes Thermal Gluons of QGP QGP Thermal Gluons  effective strangeness production process P.Koch, B. Muller and J. Rafelski: Phys.Rept.142: ,1986 In central A+A collisions, there is no phase space penalty for being strange ! There is a penalty for being heavy – exp(-m/T) ! STAR Phys. Rev. Lett. 98 (2007) 62301

Strangeness is Enhanced in A+A Collisions Or Canonically Suppressed? STAR Preliminary (Cu+Cu 200 GeV) 200 GeV Au+Au Data: Phys. Rev. C 77 (2008) 44908

5 Strangeness enhancement Strangeness enhancement: yield relative to p+p  -meson enhancement: -- between K/  and  GeV data > 62.4 GeV, unlike hyperons -- could not be solely due to the canonical suppression, there could be dynamics effect See Bedanga Mohanty’s Talk STAR Preliminary 62.4 GeV 200 GeV  K, 

6 Quark Coalescence – (ALCOR-J.Zimanyi et al, AMPT-Lin et al, Rafelski+Danos, Molnar+Voloshin …..) Quark Recombination – (R.J. Fries et al, R. Hwa et al) Hadronization of Bulk Partonic Matter  Coalescence Partons at hadronization have a v 2  Collectivity Deconfinement !

7 Phenix: PRL 98, (2007) Is KE T better variable capturing the physical picture? Empirically, maybe ! But why should it work for pions  mostly from decays why KE T  not really additive !

8  and  particles are special ! Little resonance decay contribution ! Coalescence of thermal strange quarks --- important in A+A collisions ! What is the thermal quark p T distribution ? In the hydro region – coalescence of quarks with hydro expansion OR fragmentation of quarks

99 Parton P T Distributions at Hadronization 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 feeddown contribution !  decay contribution is small These particles have small hadronic rescattering cross sections

10 Strange and down quark distributions s distribution harder than d distribution perhaps related to different s and d quarks in partonic evolution Independent Test –  /s should be consistent with s quark distribution Yes ! See Jinhui Chen’s talk

p T Scales and Physical Processes R CP Three P T Regions: -- Fragmentation -- multi-parton dynamics (recombination or coalescence or …) -- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? ) 11

12 Hydrodynamics and Coalescence Most Hydrodynamic Calculations – Cooper-Frye Freeze-out thermal statistical distribution in the co-moving frame Coalescence model – has been applied to particles with p T > 2 GeV/c or so ! For p T < 2 GeV/c  hydrodynamic behavior OR coalescence of effective constituent quarks with radial flow are these approaches equivalent ? Empirically the coalescence physical picture appealing ! Problem: -- how to deal with resonances,  effective mass of quarks ?

R AA (p T >6GeV/c) Almost p T Independent R AA =(Au+Au)/[N binary x(p+p)] Empirical Implications for a constant R AA for p T > 6 GeV/c !! 13

p T > 5 GeV/c Energy Loss Shifts pp p T to AA p T by  p T pp AA/N bin pTpT For a power-law function (1+p T /a) -n a flat R AA   p T /p T constant ! What Physical Processes? 14

Npart Dependence of Energy Loss No significant difference in  p T /p T between light hadron and non-photonic electrons !  p T /p T ~ 25% in most central collisions ! The physical origin of the N 2/3 dependence? Linear Npart not bad either 15

Absence of Explicit Path Length Dependence The centrality dependence of  p T /p T  could be due to the initial energy density in collisions ! 16

What Possible Physical Scenario for ELoss without L dependence T. Hirano et al, Phys. Rev. C69, (2004) ELoss of Partons: 1) Strong dependence on energy density 2) Rapid decrease of energy density in time interval < traversing time Hydrodynamic models show such a scenario plausible ! 17

Path-Length Dependence in Soft Particles 3<p T trig <4GeV/c & 1.0<p T asso <1.5GeV/c 20-60% STAR  =  associate -  trigger (rad) At low p T region, the medium response to Parton ELoss -- has path-length dependence Caution: the current trigger p T is high enough to be in the dominant parton energy loss domain ! 18

V 2 and R AA are Related via Path Length Dependence Precise value of v 2 at p T > 6, 10 GeV/c ? R AA at pT > 10 GeV/c at RHIC should R AA approach unity at higher p T ? Future measurements will shed lights on possible physical scenarios for parton energy loss dynamics ! Heavy Quarks will be special -- Lorentz  dependence on parton ELoss on jet-medium interaction Mach cone formation? 19

Summary 20    Central Au+Au Collisions at RHIC Bulk Partonic Matter -- 1) strangeness equilibrated 2) parton collectivity v 2 and hydro expansion 3) multi-parton dynamics coalescence/ recombination 4) p T or KE T distributions for effective quarks

Parton Energy Loss  Hadron P T Scale > 5-6 GeV/c Constant R AA   p T /p T constant as a function of p T Absence of Clear Path-Length Dependence of ELoss -- Rapid Decrease of Energy Density with Evolution Time -- Even partons originated from the center of the hot/dense fireball may escape Theoretically Eloss calculations – dynamic issue simultaneous calculation of R AA and v 2 at high p T !! 21 Summary

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Eloss ~ L*Density ? 23