Diluted equilibrium sterile neutrino dark matter
  We present a model where sterile neutrinos with rest masses in the range ~
keV to ~ MeV can be the dark matter and be consistent with all laboratory,
cosmological, large-scale structure, as well as x-ray constraints. These
sterile neutrinos are assumed to freeze out of thermal and chemical equilibrium
with matter and radiation in the very early Universe, prior to an epoch of
prodigious entropy generation ("dilution") from out-of-equilibrium decay of
heavy particles. In this work, we consider heavy, entropy-producing particles
in the ~ TeV to ~ EeV rest-mass range, possibly associated with new physics at
high-energy scales. The process of dilution can give the sterile neutrinos the
appropriate relic densities, but it also alters their energy spectra so that
they could act like cold dark matter, despite relatively low rest masses as
compared to conventional dark matter candidates. Moreover, since the model does
not rely on active-sterile mixing for producing the relic density, the mixing
angles can be small enough to evade current x-ray or lifetime constraints.
Nevertheless, we discuss how future x-ray observations, future lepton number
constraints, and future observations and sophisticated simulations of
large-scale structure could, in conjunction, provide evidence for this model
and/or constrain and probe its parameters.
