Heavy neutrino threshold effects in low energy phenomenology

Right-handed neutrinos with mass of O (1012-1013) GeV are required to implement the see-saw mechanism and generate neutrino masses capable of playing a role in structure formation. Moreover models of fermion masses often relate the Yukawa couplings involving these neutrinos to the up-quark Yukawa couplings. Here we study the effects of such couplings on the radiative corrections to quark masses. We find that b - τ equality at MGUT may still give the correct mb/mτ-ratio at low energies, but only if there is large μ - τ mixing in the charged leptonic sector. We propose specific mass matrix ''textures'' dictated by a U(1) family symmetry whose structure preserves mb=mτ at MGUT. In these schemes, due to the large νμ - ντ mixing, it is possible to give a simultaneous solution to the solar neutrino deficit and the atmospheric neutrino problem. (orig.)