Cosmology and Signals of Light Pseudo-Dirac Dark Matter
  In this paper, we analyze the cosmological evolution, allowed parameter
space, and observational prospects for a dark sector consisting of thermally
produced pseudo-Dirac fermions with a small mass splitting, coupled to the
Standard Model through a dark photon. This scenario is particularly notable in
the context of sub-GeV dark matter, where the mass-off-diagonal leading
interaction limits applicability of both CMB energy injection constraints and
standard direct detection searches. We present the first general study of the
thermal history of pseudo-Dirac DM with splittings from 100 eV to MeV, focusing
on the depletion of the heavier "excited" state abundance via scatterings and
decays, and on the distinctive signals arising from its small surviving
abundance. We analyze CMB energy injection bounds on both DM annihilation and
decay, accelerator-based probes, and new line-like direct-detection signals
from the excited DM down-scattering on either nuclei or electrons, as well as
future search prospects in each channel. We also comment on the relevance of
this signal to the few-keV Xenon1T electron excess and on possible diurnal
modulation of this signal, and introduce a signal-strength parametrization to
facilitate the comparison of future experimental results to theoretical
expectations.
