Hydrodynamical corrections to electromagnetic emissivities in QCD

TL;DR

This paper develops a framework to quantify how viscous effects in QCD fluids modify electromagnetic emission rates, highlighting the significant role of bulk viscosity near the transition temperature.

Contribution

It introduces a general method to calculate hydrodynamical corrections to QCD electromagnetic emissivities, explicitly analyzing bulk and shear viscosity effects in both hadronic and partonic phases.

Findings

Bulk viscosity significantly increases photon emission rates.

Shear viscosity effects are negligible across frequencies.

Viscous corrections are larger in the partonic phase near the transition temperature.

Abstract

We provide a general framework for the derivation of the hydrodynamical corrections to the QCD electromagnetic emissivities in a viscous fluid. Assuming that the emission times are short in comparison to the fluid evolution time, we show that the leading corrections in the fluid gradients are controlled by the bulk and shear tensors times pertinent response functions involving the energy-momentum tensor. In a hadronic fluid phase, we explicit these contributions using spectral functions. Using the vector dominance approximation, we show that the bulk viscosity correction to the photon rate is sizable, while the shear viscosity is negligible for about all frequencies. In the partonic phase near the transition temperature we provide an assessment of the viscous corrections to the photon and dilepton emissions, using a non-perturbative quark-gluon plasma with soft thermal gluonic corrections in the form of operators of leading mass dimension. Again, the thermal bulk viscosity corrections are found to be larger than the thermal shear viscosity corrections at all energies for both the photon and dilepton in the partonic phase.