Anisotropic flow of thermal photons as a quark-gluon plasma viscometer

Because photons interact only electromagnetically, photons produced in high-energy heavy-ion collisions can escape from the fireball during all collision stages, especially from its hot core. The authors demonstrate that by measuring the anisotropy of photons, one may infer the viscosity of QCD matter independently of inferring it from hadron production. This would be a relevant consistency check and lend confidence to the interpretation of the data.