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⊥≈2.5GeV and q⊥≈1.5GeV respectively. A non-vanishing viscosity leads to a larger thermalization time when extracted from the experimentally measured inverse slope (Teff) of photon q⊥ spectra. A precise, O(20MeV), measurement of photon Teff can place stringent bounds on τ0 and η/s.