Light Dark Matter through Assisted Annihilation

Abstract: In this paper we investigate light dark matter scenarios where annihilation to Standard Model particles at tree-level is kinematically forbidden. In such cases annihilation can be aided by massive Standard Model-like species, called {\it assisters}, in the initial state that enhances the available phase space opening up novel tree-level processes. We investigate the feasibility of such non-standard {\it assisted annihilation} processes to reproduce the observed relic density of dark matter. We present a simple scalar dark matter - scalar assister model where this is realised. We find that if the dark matter and assister are relatively degenerate the required relic density can be achieved for a keV-MeV scale dark matter. We briefly discuss the cosmological constraints on such dark matter scenarios.