Nonabelian dark matter models for 3.5 keV X-rays
  A recent analysis of XXM-Newton data reveals the possible presence of an
X-ray line at approximately 3.55 keV, which is not readily explained by known
atomic transitions. Numerous models of eV-scale decaying dark matter have been
proposed to explain this signal. Here we explore models of multicomponent
nonabelian dark matter with typical mass ~ 1-10 GeV (higher values being
allowed in some models) and eV-scale splittings that arise naturally from the
breaking of the nonabelian gauge symmetry. Kinetic mixing between the photon
and the hidden sector gauge bosons can occur through a dimension-5 or 6
operator. Radiative decays of the excited states proceed through transition
magnetic moments that appear at one loop. The decaying excited states can
either be primordial or else produced by upscattering of the lighter dark
matter states. These models are significantly constrained by direct dark matter
searches or cosmic microwave background distortions, and are potentially
testable in fixed target experiments that search for hidden photons. We note
that the upscattering mechanism could be distinguished from decays in future
observations if sources with different dark matter velocity dispersions seem to
require different values of the scattering cross section to match the observed
line strengths.
