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

Original · EN

We present state-of-the-art calculations of viscous photon emission from nuclear collisions at RHIC and LHC. Fluctuating initial density profiles are evolved with event-by-event viscous hydrodynamics. Momentum spectra of thermal photons radiated by these explosively expanding fireballs and their pₜ-differential anisotropic flow coefficients vₙ(pₜ) are computed, both with and without accounting for viscous corrections to the standard thermal emission rates. Viscous corrections to the rates are found to have a larger effect on the vₙ coefficients than the viscous suppression of hydrodynamic flow anisotropies. The benefits of taking the ratio of elliptic to triangular flow, v₂/v₃, are discussed, and the spacetime regions which contribute dominantly to the photon flow harmonics are identified. The directed flow v₁ of thermal photons is predicted for RHIC and LHC energies.