GeV Scale Inelastic Dark Matter with Dark Photon Mediator via Direct
  Detection and Cosmological/Laboratory Constraints
  We propose a new candidate of GeV scale inelastic dark matter (DM). Our
construction has an anomaly-free $U(1)_X^{}$ gauge group with dark photon
mediator, and can realize either scalar or fermionic inelastic DM. It is highly
predictive and testable. We study the scattering rate of light inelastic DM
with electrons in the XENON1T experiment and with nuclei in the XENON1T,
CRESST-III, CDEX-1B and DarkSide-50 experiments. We resolve the recent XENON1T
anomaly via electron recoil detection. Combining the XENON1T constraints from
both electron recoils and nuclear recoils (including Migdal effect), we predict
the inelastic DM mass $\lesssim 1.5$GeV. We further analyze the bounds by the
DM relic abundance, the lifetime of the heavier DM component, and laboratory
constraints, from which we identify the viable parameter space for the future
probe. This provides an important benchmark for the theories and experimental
tests of GeV scale inelastic DM.
