THE ONE-LOOP IMPROVED LAGRANGIAN OF THE GRIMUS-NEUFELD MODEL

Adding a single gauge singlet fermion and a second Higgs doublet to the original Standard Model allows an explanation for the observed smallness of the neutrino masses using the seesaw mechanism. This model predicts two neutral fermions with vanishing mass. The one-loop contribution to the neutral fermion masses due to the second Higgs doublet lifts this degeneracy and allows to fit the model parameters to the observed neutrino mass differences. We determine the values of the additional Yukawa couplings by requiring the correct prediction of the mass differences and mixings in the neutrino sector. We also discuss the ambiguities of the model parameters.