Neutrino mass spectrum from the seesaw extension ∗

The Standard Model includes neutrinos as massless particles, but neu-trino oscillations showed that neutrinos are not massless. A simple ex-tension of adding gauge singlet fermions to the particle spectrum allows normal Yukawa mass terms for neutrinos. The smallness of the neutrino masses can be well understood within the seesaw mechanism. We analyse two cases of the minimal extension of the standard model when one or two right-handed fields are added to the three left-handed fields. A second Higgs doublet is included in our model. We calculate the one-loop radia-tive corrections to the mass parameters which produce mass terms for the neutral leptons. In both cases we numerically analyse light neutrino masses as functions of the heavy neutrinos masses. Parameters of the model are varied to find light neutrino masses that are compatible with experimental data of solar ∆m2 and atmospheric ∆m 2 atm neutrino oscillations for normal and inverted hierarchy