Co-SIMP Miracle

Abstract

We present a new mechanism for thermally produced dark matter, based on a semi-annihilation-like process, $\chi +\chi +\text{SM}\to \chi +\text{SM}$, with intriguing consequences for the properties of dark matter. First, its mass is low, $\lesssim 1$ GeV (but $\gtrsim 5$ keV to avoid structure-formation constraints). Second, it is strongly interacting, leading to kinetic equilibrium between the dark and visible sectors, avoiding the structure-formation problems of $\chi +\chi +\chi \to \chi +\chi$ models. Third, in the $3\to 2$ process, one dark matter particle is consumed, giving the standard-model particle a monoenergetic recoil. We show that this new scenario is presently allowed, which is surprising (perhaps a "minor miracle"). However, it can be systematically tested by novel analyses in present and near-term experiments. In particular, the Co-SIMP model for thermal-relic dark matter can explain the XENON1T excess.

Comments: Minor revisions for clarity, plus a new section on how our predictions can fit the XENON1T excess. Comments are welcome