Singlet portal extensions of the standard seesaw models to a dark sector with local dark symmetry

Assuming (i) the seesaw mechanism works for neutrino masses and mixings, (ii) dark matter is absolutely stable due to unbroken U(1)_X dark gauge symmetry, and (iii) the singlet fields (H ^† H and N _R) are portal to the dark sector, we construct a simple model which is consistent with all the cosmological observations as well as terrestrial experiments available as of now, including leptogenesis, extra dark radiation of ∼ 8% (resulting in N _eff = 3.130 the effective number of neutrino species), Higgs inflation, small and large scale structure formation, and current relic density of scalar dark matter (X). The electroweak vacuum of this model is stable up to Planck scale for m _H = 125 GeV without any other new physics. The Higgs signal strength is equal to one as in the standard model for unbroken U(1) _X case with a scalar dark matter, but it could be less than one independent of decay channels if the dark matter is a dark sector fermion or if U(1) _X is spontaneously broken, because of a mixing with a new neutral scalar boson in the models. Detailed study of Higgs properties at the LHC would shed light on the models described in this work.