Discovering Inelastic Thermal-Relic Dark Matter at Colliders
  Dark Matter particles with inelastic interactions are ubiquitous in
extensions of the Standard Model, yet remain challenging to fully probe with
existing strategies. We propose a series of powerful searches at hadron and
lepton colliders that are sensitive to inelastic dark matter dynamics. In
representative models featuring either a massive dark photon or a magnetic
dipole interaction, we find that the LHC and BaBar could offer strong
sensitivity to the thermal-relic dark matter parameter space for dark matter
masses between ~100 MeV-100 GeV and fractional mass-splittings above the
percent level; future searches at Belle II with a dedicated monophoton trigger
could also offer sensitivity to thermal-relic scenarios with masses below a few
GeV. Thermal scenarios with either larger masses or splittings are largely
ruled out; lower masses remain viable yet may be accessible with other search
strategies.
