Photons as a viscometer of heavy ion collisions

TL;DR

This paper calculates how viscosity affects thermal photon production in heavy-ion collisions, enabling better extraction of medium properties like thermalization time and viscosity from photon spectra measurements.

Contribution

It provides a leading log order calculation of viscous corrections to photon production integrated with hydrodynamic evolution, improving the interpretation of photon data in heavy-ion collisions.

Findings

Viscous effects increase thermalization time estimates.

Photon spectra and flow are reliably calculable up to specified transverse momenta.

Precise photon temperature measurements can constrain collision parameters.

Abstract

The viscous correction to thermal photon production at leading log order is calculated and integrated over the space-time evolution of a hydrodynamic simulation of heavy-ion collisions. The resulting transverse momentum spectra and elliptic flow can be reliably calculated within a hydrodynamic framework up to transverse momenta of $q_\perp$ = 2.5 GeV and $q_\perp$ = 1.5 GeV respectively. A non-vanishing viscosity leads to a larger thermalization time when extracted from the experimentally measured inverse slope (T_eff) of photon $q_\perp$ spectra. A precise, O(20 MeV), measurement of photon T_eff can place stringent bounds on $\tau_0$ and $\eta/s$.