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 ν_e -> 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 (sin² 2θ_es ~ (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 ν_μ -> ν_e in the region of sensitivity of KARMEN and LSND experiments.