# Dark matter amnesia in out-of-equilibrium scenarios

## TL;DR

This paper demonstrates that in certain dark matter models with a light mediator, the complex early Universe interactions can be neglected, simplifying relic density calculations and aiding experimental constraint analysis.

## Contribution

It shows that dark matter relic density can be accurately estimated without detailed equilibration calculations in models with a light dark photon mediator.

## Key findings

- Equilibration processes often do not impact relic density significantly.
- Simplified relic density estimation is valid for models with light dark photon mediators.
- Facilitates easier superimposition of relic constraints on experimental bounds.

## Abstract

Models in which the dark matter is produced at extremely low rates from the annihilation of Standard Model particles in the early Universe allow us to explain the current dark matter relic density while easily evading the traditional experimental constraints. In scenarios where the dark matter interacts with the Standard Model via a new physics mediator, the early Universe dynamics of the dark sector can be particularly complex, as the dark matter and the mediator could be in thermal and chemical equilibrium with each other. This equilibration takes place via number-changing processes such as double Compton scattering and bremsstrahlung, whose amplitudes are cumbersome to calculate. In this paper, we show that in large regions of the parameter space, these equilibration mechanisms do not significantly affect the final dark matter relic density. In particular, for a model with a light dark photon mediator, the relic density can be reasonably estimated by considering that the dark matter is solely produced through the annihilation of Standard Model particles. This result considerably simplifies the treatment of a large class of dark matter theories, facilitating in particular the superimposition of the relic density constraints on the current and future experimental bounds.