Semi-annihilation of Dark Matter

TL;DR

This paper introduces semi-annihilation as a new process affecting dark matter relic abundance, providing a comprehensive framework and exploring its implications for early universe dynamics and indirect detection signals.

Contribution

It presents the first complete set of Boltzmann equations including semi-annihilation and analyzes toy models demonstrating its significance.

Findings

Semi-annihilation can dominate over standard annihilation in relic abundance calculations.

The process enriches the spectrum of signals for indirect dark matter detection.

Semi-annihilation introduces complex dynamics in dark matter evolution.

Abstract

We show that the thermal relic abundance of dark matter can be affected by a new type of reaction: semi-annihilation. Semi-annihilation takes the schematic form X_i X_j -> X_k phi, where X_i are stable dark matter particles and phi is an unstable state. Such reactions are generically present when dark matter is composed of more than one species with "flavor" and/or "baryon" symmetries. We give a complete set of coupled Boltzmann equations in the presence of semi-annihilations, and study two toy models featuring this process. Semi-annihilation leads to non-trivial dark matter dynamics in the early universe, often dominating over ordinary annihilation in determining the relic abundance. This process also has important implications for indirect detection experiments, by enriching the final state spectrum from dark matter (semi-)annihilation in the Milky Way.