Abstract
We present a new mechanism for thermally produced dark matter, based on a semi-annihilation-like process, χ+χ+SM→χ+SM, with intriguing consequences for the properties of dark matter. First, its mass is low, ≲1 GeV (but ≳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 χ+χ+χ→χ+χ models. Third, in the 3→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.
Highlights
Introduction.—For dark matter (DM) models, thermal production mechanisms are highly predictive frameworks [1,2,3]
The “weakly interacting massive particle (WIMP) window” is defined by the smallest mass allowed by annihilation constraints (20 GeV if neutrinos are neglected [10]; 10 MeV if they are dominant [11]) and the largest mass allowed by unitarity (150 TeV [12,13])
Recent work [18,25] has made the simple but ingenious point that the process χþχþχ → χ þ χ is efficient in the early Universe if the interactions are strong, setting the relic abundance while involving only dark-matter processes, the name strongly interacting massive particle (SIMP)
Summary
We present a new mechanism for thermally produced dark matter, based on a semi-annihilation-like process, χ þ χ þ SM → χ þ SM, with intriguing consequences for the properties of dark matter. Its mass is low, ≲1 GeV (but ≳5 keV to avoid structure-formation constraints) It is strongly interacting, leading to kinetic equilibrium between the dark and visible sectors, avoiding the structureformation problems of χþχþχ → χ þ χ models. In the 3 → 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”). Recent work [18,25] has made the simple but ingenious point that the process χþχþχ → χ þ χ is efficient in the early Universe if the interactions are strong, setting the relic abundance while involving only dark-matter processes, the name strongly interacting massive particle (SIMP). What we call the Co-SIMP mechanism leads to a dark-matter candidate with vastly different properties and phenomenology from other thermal relics.
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