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Photons as a viscometer of heavy ion collisions

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 $τ_0$ and $η/s$.