Singlet Portal Extensions of the Standard Seesaw Models to a Dark Sector
  with Local Dark Symmetry
  Assuming ({\it i}) the seesaw mechanism works for neutrino masses and
mixings, ({\it ii}) dark matter is absolutely stable due to unbroken $U(1)_X$
dark gauge symmetry, and ({\it iii}) the singlet fields ($H^\dagger H$ 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 $\sim 8 %$ (resulting in $N_{\rm eff} = 3.130$ 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.
