Dilepton production from a viscous QGP

TL;DR

This paper calculates how shear viscosity affects dilepton production in a viscous quark-gluon plasma, showing a hardening of the spectrum and providing insights into thermalization and viscosity in heavy-ion collisions.

Contribution

It introduces the first correction to dilepton production rates due to shear viscosity and integrates these into hydrodynamic simulations of RHIC collisions.

Findings

Viscous correction causes spectrum hardening

Thermal description valid for invariant masses below ~4.5 GeV

Differences in emission spectra can inform about thermalization and viscosity

Abstract

This work calculates the first correction to the leading order q\={q} dilepton production rates due to shear viscosity in an expanding gas. The modified rates are integrated over the space-time history of a viscous hydrodynamic simulation of RHIC collisions. The net result is a {\em hardening} of $q_\perp$ spectrum with the magnitude of the correction increasing with invariant mass. We argue that a thermal description is reliable for invariant masses less than $M_{max}\approx(2\tau_0 T_0^2)/(\eta/s)$. For reasonable values of the shear viscosity and thermalization time $M_{max}\approx 4.5$ GeV. Finally, the early emission from a viscous medium is compared to emission from a longitudinally free streaming plasma. Qualitative differences in $q_\perp$ spectrum are seen which could be used to extract information on the thermalization time, viscosity to entropy ratio and possibly the thermalization mechanism in heavy-ion collisions.