# Cosmic abundances of SIMP dark matter

## TL;DR

This paper refines the calculation of thermal averages for 3-to-2 dark matter annihilation processes, crucial for accurately predicting relic densities of SIMP dark matter, especially near resonance poles.

## Contribution

It provides a general method for thermal averaging of 3-to-2 and higher-order interactions, emphasizing the importance of correct averaging in models with velocity dependence and resonance effects.

## Key findings

- Correct thermal averages significantly impact relic density predictions.
- Resonance poles can alter the relic abundance calculations.
- Application to SIMP models demonstrates the importance of precise thermal averaging.

## Abstract

Thermal production of light dark matter with sub-GeV scale mass can be attributed to $3\rightarrow 2$ self-annihilation processes. We consider the thermal average for annihilation cross sections of dark matter at $3\rightarrow 2$ and general higher-order interactions. A correct thermal average for initial dark matter particles is important, in particular, for annihilation cross sections with overall velocity dependence and/or resonance poles. We apply our general results to benchmark models for SIMP dark matter and discuss the effects of the resonance pole in determining the relic density.