Naturally Stable Right-Handed Neutrino Dark Matter
  We point out that a class of non-supersymmetric models based on the gauge
group $SU(3)_C \times SU(2)_L\times SU(2)_R\times U(1)_{Y_L}\times U(1)_{Y_R}$
possesses an automatic, exact $Z_{2 }$ symmetry under which the fermions in the
$SU(2)_R\times U(1)_{Y_R}$ sector (called $R$-sector) are odd and those in the
standard model sector (called $L$-sector) are even. This symmetry, which is
different from the usual parity symmetry of the left-right symmetric models,
persists in the lepton sector even after the gauge symmetry breaks down to
$SU(3)_C \times U(1)_{\rm EM}$. This keeps the lightest right-handed neutrino
naturally stable, thereby allowing it to play the role of dark matter (DM) in
the Universe. There are several differences between the usual left-right models
and the model presented here: (i) our model can have two versions, one which
has no parity symmetry so that the couplings and masses in the $L$ and $R$
sectors are unrelated, and another which has parity symmetry so that couplings
are related; (ii) the $R$-sector fermions are chosen much heavier than the
$L$-sector ones in both scenarios; and finally (iii) both light and heavy
neutrinos are Majorana fermions with the light neutrino masses arising from a
pure type-II seesaw mechanism. We discuss the DM relic density, direct and
indirect detection prospects and associated collider signatures of the model.
Comparing with current collider and direct detection constraints, we find a
lower bound on the DM mass of order of 1 TeV. We also point out a way to relax
the DM unitarity bound in our model for much larger DM masses by an entropy
dilution mechanism. An additional feature of the model is that the DM can be
made very long lived, if desired, by allowing for weak breaking of the above
$Z_{2}$ symmetry. Our model also predicts the existence of long-lived colored
particles which could be searched for at the LHC.
