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 a 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 generations, such a heavy Dirac neutrino can be long-lived on cosmological time scales. In this model, the stated LHC exclusion range 120 GeV < m(H) < 600 GeV on the lighter Higgs placed in the context of a generic fourth-generation standard model is evaded. Also, the Dirac fourth-generation neutrino in this model, if stable, would constitute up to 1% of the cold dark matter in the Universe.