Jet quenching and direct photon production

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Jet quenching and direct photon production
F.M. Liu ??? Central China Normal
University, China T. Hirano???? University of
Tokyo, Japan K.Werner University
of Nantes, France Y. Zhu ?? Central
China Normal University, China Mainly based on
arXiv hep-ph/0807.4771v2
ATHIC2008 Tsukuba Oct 13-15
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Outline

• Motivations
• Calculation approach
• Results
• Conclusion

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Motivations

• 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.

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direct photons, jets and plasma
PRL94,232301(2005), PRL96,202301(2006)

1. 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.
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Calculation approach
A precise calculation requires careful treatments
on

• 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

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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
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Jets (hard partons)
MRST 2001 LO pDIS and EKS98 nuclear modification
are employed
Jet phase space distribution at t0
at tgt0
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Parton Energy Loss in a Plasma

• Energy loss of parton iq, 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
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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.Wangs formula
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Raa(pi0, ) at high pt gives D1.5
A common D for various Centralities!
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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
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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)

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• Results

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Centrality dependent pt-spectra(1)
PHENIX data PRL 98, 012002 (2007)
arXiv0801.4020
Our predictions coincide with the precise
measurement!
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Centrality dependent pt-spectra(2)
PRL94,232302(2005)
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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.
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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

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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.

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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 .
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Information from Thermal photons
Energy density at plasma center
Raa due to thermal source

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

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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?)
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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 pt3GeV/c and more than 70\
  at pt4GeV/c within the first
• 0.3fm/c, though the whole evolution time is
  about 20fm/c.

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Discussion and Conclusion

• Parton energy loss does make 40 decrease of
  Raa(?)
• Raa(?) 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.

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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.
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• Thank you!

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Thermal fraction