Thermal photon anisotropic flow serves as a quark–gluon plasma viscometer

Photons are a penetrating probe of the hot and dense medium created in heavy-ion collisions. We present state-of-the-art calculations of viscous photon emission from nuclear collisions at RHIC and LHC. Thermal photons' anisotropic flow coefficients vnvn are computed, both with and without accounting for viscous corrections to the standard thermal emission rates. These corrections are found to have a larger effect on the vnvn coefficients than the viscous suppression of hydrodynamic flow anisotropies. For thermal photons, the ratio v2{SP}/v3{SP}v2{SP}/v3{SP} shows stronger sensitivity to the quark–gluon plasma (QGP) shear viscosity than for hadrons, and it can thus serve as a sensitive QGP viscometer.