New Production Mechanism for keV Sterile Neutrino Dark Matter by Decays
  of Frozen-In Scalars
  We propose a new production mechanism for keV sterile neutrino Dark Matter.
In our setting, we assume the existence of a scalar singlet particle which
never entered thermal equilibrium in the early Universe, since it only couples
to the Standard Model fields by a really small Higgs portal interaction. For
suitable values of this coupling, the scalar can undergo the so-called
freeze-in process, and in this way be efficiently produced in the early
Universe. These scalars can then decay into keV sterile neutrinos and produce
the correct Dark Matter abundance. While similar settings in which the scalar
does enter thermal equilibrium and then freezes out have been studied
previously, the mechanism proposed here is new and represents a versatile
extension of the known case. We perform a detailed numerical calculation of the
DM production using a set of coupled Boltzmann equations, and we illustrate the
successful regions in the parameter space. Our production mechanism notably can
even work in models where active-sterile mixing is completely absent.
