# Prompt, pre-equilibrium, and thermal photons in relativistic nuclear collisions

## TL;DR

This paper investigates the role of pre-equilibrium photons in relativistic nuclear collisions, showing their significance at intermediate transverse momentum and their impact on photon spectra and elliptic flow, within a hydrodynamic model.

## Contribution

It introduces the inclusion of pre-equilibrium photon contributions in hydrodynamic simulations, enhancing understanding of direct photon production in nuclear collisions.

## Key findings

- Pre-equilibrium photons are relevant at intermediate transverse momentum.
- They increase particle spectra near the saturation scale.
- They reduce the elliptic flow of direct photons.

## Abstract

The direct photon emission model in relativistic nuclear colliders has been improved in recent years for reducing the discrepancy between theoretical estimations and experimental data and for understanding the properties of the QCD matter. In this study, the contribution of pre-equilibrium photons are investigated in addition to those of prompt and thermal photons in the framework of a relativistic hydrodynamic model. The numerical simulations at an LHC energy suggest that the pre-equilibrium photons may be relevant at intermediate transverse momentum near the saturation momentum scale, increasing particle spectra and reducing elliptic flow of direct photons.