Semi-annihilation of Dark Matter

TL;DR

This work introduces semi-annihilation, a DM interaction where two DM particles can produce a different DM particle plus a light state, altering thermal freeze-out beyond standard annihilation. By deriving general coupled Boltzmann equations and analyzing both a single-component $Z_3$ model and multi-component meson-baryon scenarios, the authors show that semi-annihilation can dominate relic abundance in parts of parameter space and cannot generally be captured by simple analytic reduction. They demonstrate with a minimal $bb\chi$ model that species-changing channels modify the relic density in diverse ways and require numerical treatment. For indirect detection, semi-annihilation enriches the final-state spectra via multiple mono-energetic lines, offering distinctive signatures in cosmic-ray and gamma-ray channels, even though the integrated flux may not be boosted. Overall, semi-annihilation represents a robust extension of WIMP phenomenology with concrete implications for early-universe cosmology and present-day DM searches, warranting further study of multi-component DM with stable flavor/baryon symmetries.

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.