Neutrino dark matter and the Higgs portal: improved freeze-in analysis
  Sterile neutrinos are one of the leading dark matter candidates. Their masses
may originate from a vacuum expectation value of a scalar field. If the sterile
neutrino couplings are very small and their direct coupling to the inflaton is
forbidden by the lepton number symmetry, the leading dark matter production
mechanism is the freeze-in scenario. We study this possibility in the neutrino
mass range up to 1 GeV, taking into account relativistic production rates based
on the Bose-Einstein statistics, thermal masses and phase transition effects.
The specifics of the production mechanism and the dominant mode depend on the
relation between the scalar and sterile neutrino masses as well as on whether
or not the scalar is thermalized. We find that the observed dark matter
abundance can be produced in all of the cases considered. We also revisit the
freeze-in production of a Higgs portal scalar, pointing out the importance of a
fusion mode, as well as the thermalization constraints.
