# A Non-Equilibrium Approach to Photon Emission from the Late Stages of Relativistic Heavy-Ion Collisions

## TL;DR

This paper develops a new hadronic transport approach to calculate photon emission in heavy-ion collisions, incorporating effective chiral theory and finite meson widths, to better understand photon production and flow.

## Contribution

It introduces a novel implementation of photon production cross sections into the SMASH transport model, including finite width effects, for more accurate low and intermediate energy collision simulations.

## Key findings

- Photon rates are consistent with theoretical expectations.
- Scattering via omega mesons significantly increases photon production.
- Finite rho meson width enhances low-energy photon emission.

## Abstract

Cross sections for photon production in hadronic scattering processes have been calculated according to an effective chiral field theory. For $\pi + \rho \to \pi + \gamma$ and $\pi + \pi \to \rho + \gamma$ processes, these cross sections have been implemented into a novel hadronic transport approach (SMASH), which is suitable for collisions at low and intermediate energies. The implementation is verified by systematically comparing the thermal photon rate to theoretical expectations. The photon rates we obtain are compared to previous works, where scattering processes mediated by $\omega$ mesons are found to contribute significantly to the total photon production. Finally, the impact of considering the finite width of the $\rho$ meson is investigated, and a significant enhancement of photon production in the low-energy region is observed. This work is the first step towards a consistent treatment of photon emission in hybrid hydrodynamics+transport approaches. The quantification of the importance of the hadronic stage for the resolution of the direct photon flow puzzle is a next step and can be applied to identify equilibrium and non-equilibrium effects in the hadronic afterburner.