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

TL;DR

This paper investigates how non-equilibrium dynamics influence photon production and anisotropies in the late stages of relativistic heavy-ion collisions, revealing significant effects at low transverse momentum.

Contribution

It introduces a non-equilibrium hadron transport approach to study photon production, comparing it with hydrodynamics, and highlights the impact on photon spectra and flow.

Findings

Non-equilibrium dynamics increase low p_T photon production.

Non-equilibrium dynamics enhance photon elliptic flow below 1.4 GeV.

Differences between approaches are modest in spectra but notable in flow at low p_T.

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.