Scalar Dark Matter Candidates -- Revisited
  We revisit the possibility of light scalar dark matter, in the MeV to GeV
mass bracket and coupled to electrons through fermion or vector mediators, in
light of significant experimental and observational advances that probe new
physics below the GeV-scale. We establish new limits from electron colliders
and fixed-target beams, and derive the strength of loop-induced processes that
are probed by precision physics, among other laboratory probes. In addition, we
compute the cooling bound from SN1987A, consider self-scattering, structure
formation, and cosmological constraints as well as the limits from dark
matter-electron scattering in direct detection experiments. We then show that
the combination of constraints largely excludes the possibility that the
galactic annihilation of these particles may explain the long-standing INTEGRAL
excess of 511 keV photons as observed in the galactic bulge. As caveat to these
conclusions we identify the resonant annihilation regime where the vector
mediator goes nearly on-shell.
