Neutrino mixing and see-saw mechanism

Models of neutrino masses are discussed capable of explaining in a natural way the maximal mixing between νμ and ντ observed by the Super-Kamiokande Collaboration. For three generations of leptons two classes of such models are found implying: a) Δm232≪Δm122≈Δm132 and a small mixing between νe and the other two neutrinos, b) Δm122≪Δm132≈Δm232 and a nearly maximal mixing for solar neutrino oscillations in vacuum.