One-loop corrections to the seesaw mechanism in the multi-Higgs-doublet   Standard Model
  We consider the lepton sector of the Standard Model and allow for an arbitrary number of Higgs doublets and, moreover, for the presence of right-handed neutrino singlets which enable the seesaw mechanism. In this framework, we identify and calculate the dominant one-loop radiative corrections to the tree-level mass matrix of the light neutrinos. The interesting feature is that both the tree-level and the one-loop contributions to the light-neutrino mass matrix are quadratic in the Yukawa couplings, with the effect that the one-loop contribution is smaller than the tree-level one mainly because of the one-loop factor $(16\pi^2)^{-1}$. We also point out the possibility of generating radiatively--in this framework--the ratio of solar over atmospheric neutrino mass-squared differences, as needed for the large-mixing-angle MSW solution of the solar-neutrino problem. 
W. Grimus, L. Lavoura