# Benchmarking a Non-Equilibrium Approach to Photon Emission in Relativistic Heavy-Ion Collisions

## TL;DR

This paper benchmarks a non-equilibrium hadronic transport model against theoretical and experimental photon emission rates in relativistic heavy-ion collisions, highlighting the importance of meson-mediated processes and finite width effects.

## Contribution

It introduces a new implementation of photon production cross sections into the SMASH transport model and compares results with existing theoretical and phenomenological rates.

## Key findings

- Photon production is significantly affected by meson-mediated processes.
- Finite width of the rho meson enhances low-energy photon yields.
- The approach provides a step towards consistent hybrid hydrodynamics+transport modeling.

## Abstract

In this work, the production of photons through binary scattering processes is investigated for equilibrated hadronic systems. More precisely, a non-equilibrium hadronic transport approach to describe relativistic heavy-ion collisions is benchmarked with respect to photon emission. Cross sections for photon production in $\pi + \rho \to \pi + \gamma$ and $\pi + \pi \to \rho + \gamma$ scattering processes are derived from an effective chiral field theory and implemented into the hadronic transport approach, SMASH (Simulating Many Accelerated Strongly-interacting Hadrons). The implementation is verified by systematically comparing the thermal photon rate to theoretical expectations. Further, the impact of form factors is discussed, scattering processes mediated by $\omega$ mesons are found to contribute significantly to the total photon production. Several comparisons of the yielded photon rates are performed: to parametrizations of the very same rates, as used in hydrodynamic simulations, to previous works relying on different cross sections for the production of direct photons from the hadronic stage, and to partonic rates. Finally, the impact of considering the finite width of the $\rho$ meson is investigated, where a significant enhancement of photon production in the low-energy region is observed. This benchmark is the first step towards a consistent treatment of photon emission in hybrid hydrodynamics+transport approaches and a genuine dynamical description.