Semi-annihilation of dark matter

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 psi(i)psi(j) -> psi(k)phi, where psi(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.