Models of light singlet fermion and neutrino phenomenology

We suggest that a singlet fermion S exists beyond the standard see-saw structure. It mixes with light neutrinos via interactions with the right-handed neutrino components, so that νe → S conversion solves the solar neutrino problem. Supersymmetry endowed with R-symmetry is shown to give a natural framework for existence, mass scale (∼ 3 · 10−3 eV) and mixing (sin22θes ∼ (0.1−1.5) · 10−2) of such a fermion. Models with an approximate horizontal symmetry are constructed, which embed the fermion S and explain simultaneously solar, atmospheric, hot dark matter problems as well as may predict the oscillation νμ → νe in the region of sensitivity of KARMEN and LSND experiments.