Atmospheric and solar neutrinos with a heavy singlet

We follow a minimalistic approach to neutrino masses, by introducing a single heavy singlet N into the standard model (or supersymmetric standard model) with a heavy Majorana mass M, which couples as a single right-handed neutrino in a Dirac fashion to leptons, and induces a single light see-saw mass m_ν~5x10^-2 eV, leaving two neutrinos massless. This trivial extension to the standard model may account for the atomospheric neutrino data via ν_μ-->ν_τ oscillations with near maximal mixing angle θ₂₃~π/4 and Δm_μτ²~2.5x10^-3 eV². In order to account for the solar neutrino data the model is extended to SUSY GUT/ string-inspired type models which can naturally yield an additional light tau neutrino mass m_ντ~fewx10^-3 eV leading to ν_eL-->(cosθ₂₃ν_μL- sinθ₂₃ν_τL) oscillations with Δm_e1²~10^-5 eV² and a mixing angle sin²2θ₁~10^-2 in the correct range for the small angle MSW solution to the solar neutrino problem. The model predicts ν_eL-- >(sinθ₂₃ν_μL+cosθ₂₃ν_τL) oscillations with a similar angle but a larger splitting Δm_e2²~2.5x10^-3 eV².