Light(ly)-coupled Dark Matter in the keV Range: Freeze-In and
  Constraints
  Dark matter produced from thermal freeze-out is typically restricted to have
masses above roughly 1 MeV. However, if the couplings are small, the freeze-in
mechanism allows for production of dark matter down to keV masses. We consider
dark matter coupled to a dark photon that mixes with the photon and dark matter
coupled to photons through an electric or magnetic dipole moment. We discuss
contributions to the freeze-in production of such dark matter particles from
standard model fermion-antifermion annihilation and plasmon decay. We also
derive constraints on such dark matter from the cooling of red giant stars and
horizontal branch stars, carefully evaluating the thermal processes as well as
the bremsstrahlung process that dominates for masses above the plasma
frequency. We find that the parameters needed to obtain the observed relic
abundance from freeze-in are excluded below a few tens of keV, depending on the
value of the dark gauge coupling constant for the dark photon portal model, and
below a few keV, depending on the reheating temperature for dark matter with an
electric or magnetic dipole moment. While laboratory probes are unlikely to
probe these freeze-in scenarios in general, we show that for dark matter with
an electric or magnetic dipole moment and for dark matter masses above the
reheating temperature, the couplings needed for freeze-in to produce the
observed relic abundance can be probed partially by upcoming direct-detection
experiments.
