Axion-Assisted Production of Sterile Neutrino Dark Matter
  Sterile neutrinos can be generated in the early universe through oscillations
with active neutrinos and represent a popular and well-studied candidate for
our universe's dark matter. Stringent constraints from X-ray and gamma-ray line
searches, however, have excluded the simplest of such models. In this letter,
we propose a novel alternative to the standard scenario in which the mixing
angle between the sterile and active neutrinos is a dynamical quantity, induced
through interactions with a light axion-like field. As the energy density of
the axion-like particles is diluted by Hubble expansion, the degree of mixing
is reduced at late times, suppressing the decay rate and easily alleviating any
tension with X-ray or gamma-ray constraints. We present a simple model which
illustrates the phenomenology of this scenario, and also describe a framework
in which the QCD axion is responsible for the production of sterile neutrinos
in the early universe.
