Strange Probes of QCD Matter

Huan Zhong Huang

Department of Physics and Astronomy

University of California

Los Angeles, CA 90095-1547

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) PT Scale for Jet Energy Loss in QCD Medium

5) Is There a Clear Path-Length Effect in Eloss?

6) Ourlook

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Strangeness Probes Thermal Gluons of QGPQGP Thermal Gluons effective strangeness production process P.Koch, B. Muller and J. Rafelski: Phys.Rept.142:167-262,1986

In central A+A collisions, there is no phase space penalty for being strange !

There is a penalty for being heavy – exp(-m/T) !

Au+Au@200GeV STARPhys. 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

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Strangeness enhancement

Strangeness enhancement: yield relative to p+p

-meson enhancement:

-- between K/ and

-- 200 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,

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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 v2

Collectivity Deconfinement !

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Phenix: PRL 98, 162301 (2007)

Is KET better variable capturing the physical picture?

Empirically, maybe ! But why should it work for pions mostly from decays why KET not really additive !

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Cu+Cu@200GeVCu+Cu@200GeV

Au+Au@200GeV

and particles are special !

Little resonance decay contribution !

Coalescence of thermal strange quarks --- important in A+A

collisions !

What is the thermal quark pT distribution ?

In the hydro region – coalescence of quarks with hydro expansionOR fragmentation of quarks

99

Parton PT Distributions at HadronizationIf 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

)2/(

)3/(

)2/(

)3/(

T

T

T

T

p

pd

p

ps

and particles have no decay feeddown contribution ! decay contribution is smallThese particles have small hadronic rescattering cross sections

1010

Strange and down quark distributions

s distribution harder than d distributionperhaps 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

pT Scales and Physical Processes

RCPThree PT Regions:

-- Fragmentation

-- multi-parton dynamics (recombination or coalescence or …)

-- Hydrodynamics (constituent quarks ? parton dynamics from gluons to constituent quarks? )

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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 pT > 2 GeV/c or so !

For pT < 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 ?

RAA(pT>6GeV/c) Almost pT Independent

RAA=(Au+Au)/[Nbinaryx(p+p)]

Empirical Implications for a constant RAA for pT > 6 GeV/c !!13

pT > 5 GeV/c

Energy Loss Shifts pp pT to AA pT by pT

pp

AA/Nbin

pT

For a power-law function (1+pT/a)-n a flat RAA pT/pT constant ! What Physical Processes?

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Npart Dependence of Energy Loss

No significant difference in pT/pT between light hadron and non-photonic electrons !pT/pT ~ 25% in most central collisions !The physical origin of the N2/3 dependence? Linear Npart not bad either

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Absence of Explicit Path Length Dependence

The centrality dependence of pT/pTcould be due to the initial energy density in collisions !

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What Possible Physical Scenario for ELoss without L dependence

T. Hirano et al, Phys. Rev. C69, 034908 (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 !

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Path-Length Dependence in Soft Particles

3<pTtrig<4GeV/c & 1.0<pT

asso<1.5GeV/c 20-60%STAR

= associate - trigger (rad)

At low pT region, the medium response to Parton ELoss -- has path-length dependenceCaution: the current trigger pT is high enough to be in the dominant parton energy loss domain !

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V2 and RAA are Related via Path Length Dependence

Precise value of v2 at pT > 6, 10 GeV/c ?

RAA at pT > 10 GeV/c at RHIC should RAA approach unity at higher pT ?

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?

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Summary

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Central Au+Au Collisions at RHIC Bulk Partonic Matter --

1) strangeness equilibrated

2) parton collectivityv2 and hydro expansion

3) multi-parton dynamicscoalescence/recombination

4) pT or KET distributionsfor effective quarks

Parton Energy Loss Hadron PT Scale > 5-6 GeV/c

Constant RAA pT /pT constant as a function of pT

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 RAA and v2 at high pT !!

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Summary

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Eloss ~ L*Density ?

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