Mechanism for Thermal Relic Dark Matter of Strongly Interacting Massive Particles

We present a new paradigm for achieving thermal relic dark matter. The mechanism arises when a nearly secluded dark sector is thermalized with the standard model after reheating. The freeze-out process is a number-changing 3 -> 2 annihilation of strongly interacting massive particles (SIMPs) in the dark sector, and points to sub-GeV dark matter. The couplings to the visible sector, necessary for maintaining thermal equilibrium with the standard model, imply measurable signals that will allow coverage of a significant part of the parameter space with future indirect-and direct-detection experiments and via direct production of dark matter at colliders. Moreover, 3 -> 2 annihilations typically predict sizable 2 -> 2 self-interactions which naturally address the core versus cusp and too-big-to-fail small-scale structure formation problems.