1. 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

2. Outline Strangeness in Bulk Partonic Matter
Hadronization and Evolution Dynamics
Thermalized Effective Quarks
PT Scale for Jet Energy Loss in QCD Medium
Is There a Clear Path-Length Effect in Eloss?
Ourlook

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 !
STAR
Phys. Rev. Lett. 98 (2007) 62301
There is a penalty for being heavy – exp(-m/T) !

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

5. Strangeness enhancement
STAR Preliminary
62.4 GeV
200 GeV X
K, L
Strangeness enhancement: yield relative to p+p
-meson enhancement:
-- between K/L and X
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

6. Hadronization of Bulk Partonic Matter  Coalescence
Partons at hadronization
have a v2
 Collectivity
Deconfinement !
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)

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

8. W and f 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 expansion
OR fragmentation of quarks

9. Parton PT Distributions at Hadronization
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 feeddown contribution !
decay contribution is small
These particles have small hadronic rescattering cross sections 9

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 –
f/s should be consistent with s quark distribution
Yes ! 10
See Jinhui Chen’s talk

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

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
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, r w
effective mass of quarks ?

13. RAA(pT>6GeV/c) Almost pT Independent
RAA=(Au+Au)/[Nbinaryx(p+p)]
Empirical Implications for a constant RAA for pT > 6 GeV/c !!

14. Energy Loss Shifts pp pT to AA pT by DpT
AA/Nbin
DpT
pT > 5 GeV/c
For a power-law function (1+pT/a)-n
a flat RAA  DpT/pT constant !
What Physical Processes?

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

16. Absence of Explicit Path Length Dependence
The centrality dependence of DpT/pT could be due to
the initial energy density in collisions !

17. 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 !

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

19. 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 g dependence
on parton ELoss
on jet-medium interaction
Mach cone formation?

20. Summary L X W Central Au+Au Collisions at RHIC Bulk Partonic Matter --
1) strangeness equilibrated
2) parton collectivity
v2 and
hydro expansion
3) multi-parton dynamics
coalescence/
recombination
4) pT or KET distributions
for effective quarks W X L

21. Summary Parton Energy Loss  Hadron PT Scale > 5-6 GeV/c
Constant RAA  DpT /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 !!

23. Eloss ~ L*Density ?