Model for Thermal Relic Dark Matter of Strongly Interacting Massive Particles
A recent proposal is that dark matter could be a thermal relic of $3\ensuremath{\rightarrow}2$ scatterings in a strongly coupled hidden sector. We present explicit classes of strongly coupled gauge theories that admit this behavior. These are QCD-like theories of dynamical chiral symmetry breaking, where the pions play the role of dark matter. The number-changing $3\ensuremath{\rightarrow}2$ process, which sets the dark matter relic abundance, arises from the Wess-Zumino-Witten term. The theories give an explicit relationship between the $3\ensuremath{\rightarrow}2$ annihilation rate and the $2\ensuremath{\rightarrow}2$ self-scattering rate, which alters predictions for structure formation. This is a simple calculable realization of the strongly interacting massive-particle mechanism.