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 image and image respectively. A non-vanishing viscosity leads to a larger thermalization time when extracted from the experimentally measured inverse slope (image) of photon image spectra. A precise, image, measurement of photon image can place stringent bounds on image and image.