Abelian Extension of Standard Model with Four Generations
  An abelian gauge extension of the Standard Model is proposed with a fourth generation. The fourth generation fermions obtain their masses from a heavier Higgs doublet which makes no tree level contributions to the first three generations masses. Light first three generations neutrino masses continue to have type I seesaw explanation whereas the fourth generation neutrino turns out to be a heavy Dirac neutrino. In the minimal version of such a model with no off-diagonal Yukawa couplings between the fourth and the first three generation, such a heavy Dirac neutrino can be long lived on cosmological time scales. In this model the stated LHC exclusion range $120 \; \text{GeV} < m_H < 600 \; \text{GeV}$ on the lighter Higgs placed in the context of generic fourth generation standard model is evaded. Also, the Dirac fourth generation neutrino in this model, if stable would constitute upto 1% of the cold Dark Matter in the Universe. 
Debasish Borah
hep-ph