Models for Self-resonant Dark Matter
  We consider a new mechanism for enhancing the self-scattering and
annihilation cross sections for dark matter with multiple components but
without a light mediator. The lighter dark matter component plays a role of the
$u$-channel pole in the elastic co-scattering for dark matter, leading to a
large self-scattering cross section and a Sommerfeld enhancement for
semi-annihilation processes. Taking the effective theory approach for
self-resonant dark matter, we present various combinations of multiple dark
matter components with spins and parities, showing a $u$-channel pole in the
co-scattering processes. Adopting dark photon and dark Higgs portals for
self-resonant dark matter, we impose the relic density condition as well as
indirect detection bounds on semi-annihilation channels with a Sommerfeld
enhancement and discuss potential signals for direct detection experiments.
