Dark matter, singlet extensions of the nuMSM, and symmetries
  We consider an extension of the nuMSM in which sterile neutrino masses
originate from the VEV of a Higgs singlet phi and dark matter is produced
through the decays of phi rather than through active-sterile neutrino mixing.
This model, which we refer to as the nuNMSM, can readily satisfy or escape the
constraints on warm dark matter from the Lyman-alpha forest and other small
scale structure. However, it requires a particular hierarchy of Majorana masses
and Yukawa couplings without an obvious origin. We show that the hierarchical
parameters of the nuNMSM can arise from symmetries broken at or near the Planck
scale for two specific examples of this model: one in which phi helps stabilize
the electroweak vacuum through a scalar threshold effect and one in which phi
is a light inflaton. Both examples require a complex phi and have several
experimental signatures that are distinct from the nuMSM. These signatures
include additional dark radiation that is relativistic at both primordial
nucleosynthesis and CMB decoupling and, for the former, a large invisible
branching ratio of the Higgs.
