Out-of-equilibrium photon production in the late stages of relativistic heavy-ion collisions

Abstract

In this work, we assess the importance of nonequilibrium dynamics in the production of photons from the late stages of relativistic heavy-ion collisions. The ${p}_{\mathrm{T}}$-differential spectra and ${v}_{2}$ of photons from the late hadronic stage are computed within a nonequilibrium hadron transport approach, and compared to the results of a local equilibrium evolution using ideal relativistic hydrodynamics. It is found that nonequilibrium dynamics enhance the late-stage photon production at low ${p}_{\mathrm{T}}$ and decreases it at higher ${p}_{\mathrm{T}}$ compared to the estimate from hydrodynamics. This same comparison points to a significant increase in the momentum anisotropies of these photons due to nonequilibrium dynamics. Once combined with photons produced above the particlization temperature in the hydrodynamics evolution, the differences between the two approaches appear modest for the ${p}_{\mathrm{T}}$ differential spectra, but are clearly noticeable at low ${p}_{\mathrm{T}}$ for the elliptic flow: nonequilibrium dynamics enhance the photon ${v}_{2}$ below ${p}_{\mathrm{T}} \ensuremath{\approx}1.4$ GeV.