# Photons from the Early Stages of Relativistic Heavy Ion Collisions

## TL;DR

This paper investigates photon production in early stages of relativistic heavy ion collisions, showing that early photons significantly contribute to the photon spectrum and indicating no dark age in such collisions.

## Contribution

It introduces a novel calculation of early stage photon contribution using classical gluon fields and kinetic theory, highlighting their importance in the photon spectrum.

## Key findings

- Early stage photons significantly contribute at p_T ≈ 2 GeV and above.
- The amount of early photons is comparable to thermalized plasma photons.
- Early photons serve as a signature of the initial collision stage.

## Abstract

We present results about photons production in relativistic heavy ion collisions. The main novelty of our study is the calculation of the contribution of the early stage photons to the photon spectrum. The initial stage is modeled by an ensemble of classical gluon fields which decay to a quark-gluon plasma via the Schwinger mechanism, and the evolution of the system is studied by coupling classical field equations to relativistic kinetic theory; photons production is then computed by including the pertinent collision processes into the collision integral. We find that the contribution of the early stage photons to the direct photon spectrum is substantial for $p_T \approx 2$ GeV and higher, the exact value depending on the collision energy; therefore we identify this part of the photon spectrum as the sign of the early stage. Moreover, the amount of photons produced during the early stage is not negligible with respect to those produced by a thermalized quark-gluon plasma: we support the idea that there is no dark age in relativistic heavy ion collisions.