Out-of-Equilibrium Photon Production in the Late Stages of Relativistic Heavy-Ion Collisions

Abstract

In this work, we assess the importance of non-equilibrium 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 non-equilibrium hadron transport approach, and compared to the results of a local equilibrium evolution using ideal relativistic hydrodynamics. It is found that non-equilibrium 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 non-equilibrium dynamics. Once combined with photons produced above the particlization temperature in the hydrodynamics evolution, the differences between the two approaches appear modest in what concerns the p${}_{\mathrm{T}}$ differential spectra, but are clearly noticeable at low p${}_{\mathrm{T}}$ for the elliptic flow: non-equilibrium dynamics enhance the photon $v_2$ below p${}_{\mathrm{T}}$ $\approx 1.4$ GeV.

Comments: Data is available in the anc directory