Enabling forbidden dark matter

Abstract

The thermal relic density of dark matter is conventionally set by two-body annihilations. We point out that in many simple models, $3\ensuremath{\rightarrow}2$ annihilations can play an important role in determining the relic density over a broad range of model parameters. This occurs when the two-body annihilation is kinematically forbidden, but the $3\ensuremath{\rightarrow}2$ process is allowed; we call this scenario not-forbidden dark matter. We illustrate this mechanism for a vector-portal dark matter model, showing that for a dark matter mass of ${m}_{\ensuremath{\chi}}\ensuremath{\sim}\mathrm{MeV}\text{\ensuremath{-}}10\text{ }\text{ }\mathrm{GeV}$, $3\ensuremath{\rightarrow}2$ processes not only lead to the observed relic density, but also imply a self-interaction cross section that can solve the cusp/core problem. This can be accomplished while remaining consistent with stringent CMB constraints on light dark matter, and can potentially be discovered at future direct detection experiments.