Thermal photons in QGP and non-ideal effects

TL;DR

This paper studies how non-ideal effects like bulk viscosity and realistic equations of state influence thermal photon production in quark-gluon plasma using relativistic hydrodynamics, highlighting significant impacts near the phase transition.

Contribution

It incorporates recent lattice QCD estimates into hydrodynamic modeling to assess non-ideal effects on photon emission, including cavitation phenomena and viscous corrections.

Findings

Non-ideal gas behavior increases photon production rates.

Bulk viscosity effects are smaller than non-ideal equation of state effects.

Ignoring cavitation can lead to inaccurate photon flux estimates.

Abstract

We investigate the thermal photon production-rates using one dimensional boost-invariant second order relativistic hydrodynamics to find proper time evolution of the energy density and the temperature. The effect of bulk-viscosity and non-ideal equation of state are taken into account in a manner consistent with recent lattice QCD estimates. It is shown that the \textit{non-ideal} gas equation of state i.e $\epsilon-3\,P\,\neq 0$ behaviour of the expanding plasma, which is important near the phase-transition point, can significantly slow down the hydrodynamic expansion and thereby increase the photon production-rates. Inclusion of the bulk viscosity may also have similar effect on the hydrodynamic evolution. However the effect of bulk viscosity is shown to be significantly lower than the \textit{non-ideal} gas equation of state. We also analyze the interesting phenomenon of bulk viscosity induced cavitation making the hydrodynamical description invalid. We include the viscous corrections to the distribution functions while calculating the photon spectra. It is shown that ignoring the cavitation phenomenon can lead to erroneous estimation of the photon flux.