Neutrino mixing and see-saw mechanism

Models of neutrino masses are discussed capable of explaining in a natural way the maximal mixing between ν_μ and ν_τ observed by the Super-Kamiokande Collaboration. For three generations of leptons two classes of such models are found implying: a) Δm₂₃²Δm₁₂²≈Δm₁₃² and a small mixing between ν_e and the other two neutrinos, b) Δm₁₂²Δm₁₃²≈Δm₂₃² and a nearly maximal mixing for solar neutrino oscillations in vacuum.