# Münster 2017 – wissenschaftliches Programm

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# T: Fachverband Teilchenphysik

## T 41: Beyond the Standard Model (Theorie) 2

### T 41.6: Vortrag

### Dienstag, 28. März 2017, 12:15–12:30, VSH 19

**Departure from chemical equilibrium in dark matter co-annihilation** — Mathias Garny^{1}, Jan Heisig^{2}, •Benedikt Lülf^{2}, and Stefan Vogl^{3} — ^{1}Technical University Munich — ^{2}RWTH Aachen — ^{3}Max-Planck-Institut für Kernphysik, Heidelberg

The observed dark matter density may be explained by the presence of a thermal relic which is often assumed to be a weakly interacting massive particle freezing out during an early stage of the hot universe. In theories beyond the standard model, like e.g. supersymmetry, regions with co-annihilations are of particular interest. They potentially allow us to reconcile the tightening limits from dark matter searches and the explanation of the observed relic density. In general, the evolution of the involved particle densities are described by a system of coupled Boltzmann equations. The standard method of solving these equations assumes chemical equilibrium between dark matter and co-annihilating particles. This assumption allows us to reduce the coupled system of differential equations to a single Ricatti-like equation by summing over the whole sector of annihilating particles. The equation can then be solved numerically or e.g. in the freeze-out approximation. In this work, we investigate the validity of the assumption of chemical equilibrium during freeze-out and show examples where it breaks down and hence requires the solution of the full set of coupled differential equations. In particular, in the framework of a simplified dark matter model, we find examples where the standard method (assuming chemical equilibrium) undershoots the correct prediction for the relic density by orders of magnitude.