Dimension-five effective operators in electro-weak SU(4)xU(1) gauge   models
  We prove in this paper that the electro-weak SU(4)L x U(1)X gauge models with spontaneous symmetry breaking can offer a natural framework for generating neutrino masses by simply exploiting the tree level realization of dimension-five effective operators. The novelty of our approach resides in the fact that the scalar sector needs not to be enlarged, since these operators are constructed as direct products among scalar multiplets already existing in the model. There is a unique generic matrix for Youkawa couplings in the neutrino sector. The charged leptons are already in their diagonal basis. This framework can lead to a suitable fit of the established phenomenology for the left-handed neutrinos, while the right-handed neutrino masses come out in the sub-keV region, independently of the cut-off. The latter introduces in the theory an intermediate scale (however, more close to GUT than to SM) at about 10^12GeV which is a crucial ingredient for the left-handed neutrino phenomenology. 
Adrian Palcu
hep-ph