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Jet quenching and direct photon production. F.M. Liu 刘复明 Central China Normal University, China T. Hirano 平野哲文 University of Tokyo, Japan K.Werner University of Nantes, France Y. Zhu 朱燕 Central China Normal University, China Mainly based on arXiv: hep-ph/0807.4771v2.

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jet quenching and direct photon production

Jet quenching and direct photon production

F.M. Liu 刘复明Central China Normal University, China

T. Hirano平野哲文University of Tokyo, Japan

K.Werner University of Nantes, France

Y. Zhu 朱燕Central China Normal University, China

Mainly based on arXiv: hep-ph/0807.4771v2

ATHIC2008 Tsukuba Oct 13-15

  • Motivations
  • Calculation approach
  • Results
  • Conclusion
  • Heavy ion collisions at various centralities offer us various bulks of hot dense matter.
  • The interaction between jets ( hard partons) and the bulk has received notable interest, i.e. jet quenching is one of the most exciting observables at RHIC.
  • The interaction of partons inside the bulk and the properties of the bulk are of great interest, which may offer us some insight to quark confinement.
direct photons jets and plasma
direct photons, jets and plasma

PRL94,232301(2005), PRL96,202301(2006)

  • Jet queching gives different effects to direct photons?

2. Direct photons (thermal, jet-photon conversion) are penetrating probes for the interaction of partons inside the bulk and the interaction between jet and bulk. We can make cross check of the properties of the medium.

calculation approach
Calculation approach
  • The space-time evolution of the created hot dense matter
  • The propagation of jets in plasma

( interaction between jet + plasma)

  • All sources of direct photons

A precise calculation requires careful treatments on

space time evolution of plasma
Space-time evolution of Plasma

Described with ideal hydrodynamics in full 3D space

Constrained with PHOBOS data

Tested with hadrons’ yields, spectra, v2 and particles correlation

For more details, read T. Hirano

jets hard partons
Jets (hard partons)

MRST 2001 LO pDIS and EKS98 nuclear modification are employed

Jet phase space distribution at τ=0:

at τ>0:

parton energy loss in a plasma
Parton Energy Loss in a Plasma
  • Energy loss of parton i=q, g, D: free parameter
  • Energy loss per unit distance, with BDMPS
  • Every factor depends on the location of jet in plasma , i.e.,

fQGP: fraction of QGP at a given point

fix d with pi0 suppression
Fix D with pi0 suppression
  • From pp collisions:
  • From AA collisions, parton energy loss is considered

via modified fragmentation function

Factorization scale and renormalization scale to be

X.N.Wang’s formula

raa pi0 at high pt gives d 1 5
Raa(pi0, %) at high pt gives D=1.5

A common D

for various


sources of direct photons
Sources of direct photons
  • Leading Order contr. from primordial NN scatterings
  • Thermal contribution

Interactions of thermal partons

are inside the rate!

Coupling depends on temperature

sources of direct photons1
Sources of direct photons
  • Jet photon conversion
  • Fragmentation contribution: similar to pi0 production

Ignored contributions: Medium induced radiation (mainly at low pt )

radiation from pre-equilibrium phase (short time)

centrality dependent pt spectra 1
Centrality dependent pt-spectra(1)

PHENIX data: PRL 98, 012002 (2007) & arXiv:0801.4020

Our predictions coincide with the precise measurement!

pt spectrum from pp collisions
Pt spectrum from pp collisions

PRL 98, 012002 (2007)

A good test for contributions

from leading order +

fragmentation without Eloss

in AA collisions.

The PHENIX fit of pp spectrum is used for Raa of thermal photons.

raa energy loss
Raa: energy loss

PRL94,232302(2005);J.Phys.G34, S1015-1018,2007

  • Data is reproduced within theory uncertainty.
  • E loss makes about 40% decrease of total photon production
  • Centrality independent ? central and peripheral results differ
  • by less than 5% with Eloss
  • by about 20% at intermediate pt w/o Eloss
centrality dependent suppression
Centrality-dependent suppression
  • E loss does play a important role in fragmentation contribution and
  • jet photon-conversion contribution.
  • This is centrality-dependent, similar to the suppression to pi0 production.
competition btw different sources
Competition btw different sources

Thermal and LO dominate low and high pt region respectively.

Raa is not sensitive to E loss, because of the centrality dependence of them.

When collisions move to perpherial, JPC becomes less important

while fragmentation becomes more important .

information from thermal photons
Information from Thermal photons

Raa due to thermal source

Energy density at plasma center

  • High Temp. from fitting pt spectrum  A higher Temp. plasma
  • More yields (shines) of thermal photons  A bigger-size (longer-life) plasma.
v2 of thermal photons
V2 of thermal photons

Contrary to hadronic v2 (ideal hydro predicted increase monotonically),

the elliptic flow of thermal photons decrease at high pt!

( Information for the earlier evolution of the plasma?)

time evolution
Time evolution
  • At initial time there is no transverse flow, so v2 vanishes.
  • A big fraction of energetic thermal photons are emitted at early time:
  • More than 50\% at pt=3GeV/c and more than 70\% at pt=4GeV/c within the first
  • 0.3fm/c, though the whole evolution time is about 20fm/c.
discussion and conclusion
Discussion and Conclusion
  • Parton energy loss does make 40% decrease of Raa(γ)
  • Raa(γ) is independent of centrality (within 5% accuracy) because of

1) the dominance of leading order contribution

2) strong suppression to JPC and frag. contributions due to E-loss

  • Thermal photons can provide information of the temperature and size of the plasma via the slope of pt spectrum and the yields.
  • The elliptic flow of thermal photons is predicted to first increase and then decrease with pt, contrary to hadronic v2, which does not carry the early information of the QGP.
r aa suppression from initial effect
RAA suppression from initial effect

The dominant contribution at high pt is the LO contribution from NN collisions:

Isosping mixture and nuclear shadowing:

The isospin mixture

and nuclear shadowing

reduce Raa at high pt.

This is the initial effect, not

related to QGP formation.