Thermal radiation from QGP and Lattice QCD

1
Thermal radiation from QGP and Lattice
QCD
Péter Petreczky
RIKEN-BNL and Physics
Department Brookhaven
National Laboratory
Production rate of thermal photons and dileptos
spectral functions
Lattice QCD calculations
Current-current correlator in Eucledian time
Motivations coupling constant is large close
to but even at very high temperatures
non-perturbative effects are important at the
momentum scale
AGS and RHIC Users Meeting, BNL, June 5, 2006
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Meson correlators and spectral functions
Thermal dilepton rate
Thermal photon rate
McLerran, Toimela, PRD 31 (85) 545
LQCD
MEM
3
Lattice setup
Quenched QCD with Wilson fermions
or Staggered fermions
Isotropic Lattice
Anisotropic Lattice
time
space
space
Karsch, Laermann, Petreczky, Stickan, Wetzorke,
PLB 530 (02) 147 Non-perturbatively impr. Wilson
action
Asakasa, Hatsuda, Nakahara, NPA 715 (03) 863
Std. Wilson action

4
Reconstruction of the spectral functions
data and
degrees of freedom to reconstruct
Bayesian techniques find
which maximizes
data
Prior knowledge
Maximum Entropy Method (MEM)
Asakawa, Hatsuda, Nakahara, PRD 60 (99) 091503,
Prog. Part. Nucl. Phys. 46 (01) 459
Likelihood function
Shannon-Janes entropy
-perturbation theory
- default model
5
Numerical results on the correlator of the vector
currents
Lattice spacing dependence is small !
constraints on the spectral functions at small
energies
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Vector spectral functions above deconfinement
Karsch, Laermann, Petreczky, Stickan, Wetzorke,
PLB 530 (02) 147
Asakasa, Hatsuda, Nakahara, NPA 715 (03) 863
??
lattice artifacts
Mesons below
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Spectral function and thermal dilepton rate
Karsch, Laermann, Petreczky, Stickan, Wetzorke,
PLB 530 (02) 147
suppression of low mass dileptons
in sharp contradiction with perturbative
expectations predicting enhancement at low
energy Braaten, Pisarski, Yuan, PRL 64 (90) 2242
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Spectral functions at finite momenta
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Spectral functions at finite momenta with
staggered fermions
Aarts et al , Strong and Electroweak Matter 2006
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Removing lattice artifacts in the spectral
functions
Free spectral functions
Wilson fermions
Highly improved (HYP) fermions
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Removing lattice artifacts in the spectral
functions
Free spectral functions
Wilson fermions
Highly improved (HYP) fermions
Karsch, Laermann, Petreczky, Stickan, Phys. Rev.
D68 (03) 014504
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Summary

• Lattice calculations of the vector current
  correlators together
• with MEM can provide some informations about
  thermal radiation

• Calculations with Wilson fermions suggest
  suppression of low mass thermal
• dileptons compared to the naïve perturbative
  predictions and
• possibly very small photon rate
• Results with Wilson fermions versus staggered
  fermions ??

• Lattice artifacts in these calculations are a
  problem they are
• under control at the level of the correlation
  functions but need
• to be understood in the spectal functions gt
  highly improved actions