Dark Matter Asymmetry from Semi-annihilation

We investigate a possible particle-antiparticle asymmetry in the dark sector via a general semi-annihilation process, in which two dark particles (antiparticles) annihilate to a dark antiparticle (particle) and a standard model-like state. We show that the semi-annihilation with maximal CP-violation can generate a completely asymmetric relic density of dark matter (DM) without requiring a subsequent pair annihilation process. We also find that the upper bound on the DM mass from S-matrix unitarity in the purely asymmetric semi-annihilation scenario, is around 15 GeV, which is much stronger than the standard WIMP scenario.