# Coannihilation without chemical equilibrium

## TL;DR

This paper introduces a new conversion-driven freeze-out mechanism for dark matter, where chemical equilibrium fails, leading to relic densities governed by conversion rates rather than self-annihilation, with distinctive collider signatures.

## Contribution

It demonstrates a novel freeze-out process without chemical equilibrium, expanding the understanding of dark matter relic density determination beyond traditional assumptions.

## Key findings

- Identifies parameter regions with small couplings matching observed relic density.
- Shows freeze-out occurs out of chemical equilibrium with inefficient self-annihilation.
- Predicts collider signatures like disappearing tracks or displaced vertices.

## Abstract

Chemical equilibrium is a commonly made assumption in the freeze-out calculation of coannihilating dark matter. We explore the possible failure of this assumption and find a new conversion-driven freeze-out mechanism. Considering a representative simplified model inspired by supersymmetry with a neutralino- and sbottom-like particle we find regions in parameter space with very small couplings accommodating the measured relic density. In this region freeze-out takes place out of chemical equilibrium and dark matter self-annihilation is thoroughly inefficient. The relic density is governed primarily by the size of the conversion terms in the Boltzmann equations. Due to the small dark matter coupling the parameter region is immune to direct detection but predicts an interesting signature of disappearing tracks or displaced vertices at the LHC. Unlike freeze-in or superWIMP scenarios, conversion-driven freeze-out is not sensitive to the initial conditions at the end of reheating.