Electromagnetic Signals of Inelastic Dark Matter Scattering
  Light dark sectors in thermal contact with the Standard Model naturally
produce the observed relic dark matter abundance and are the targets of a broad
experimental search program. A key light dark sector model is the pseudo-Dirac
fermion with a dark photon mediator. The dynamics of the fermionic excited
states are often neglected. We consider scenarios in which a nontrivial
abundance of excited states is produced and their subsequent de-excitation
yields interesting electromagnetic signals in direct detection experiments. We
study three mechanisms of populating the excited state: a primordial excited
fraction, a component up-scattered in the sun, and a component up-scattered in
the Earth. We find that the fractional abundance of primordial excited states
is generically depleted to exponentially small fractions in the early universe.
Nonetheless, this abundance can produce observable signals in current dark
matter searches. MeV-scale dark matter with thermal cross sections and higher
can be probed by down-scattering following excitation in the sun. Up-scatters
of GeV-scale dark matter in the Earth can give rise to signals in current and
upcoming terrestrial experiments and X-ray observations. We comment on the
possible relevance of these scenarios to the recent excess in XENON1T.
