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 $\n_e \to S$ conversion solves the solar neutrino problem. Supersymmetry endowed with R-symmetry is shown to give a natural framework for existence, mass scale and mixing ($\sin^22\th_{es} \sim (0.1-1.5)\cdot 10^{-2}$) 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 $\bar{\n}_\m \to \bar{\n}_e$ in the region of sensitivity of KARMEN and LSND experiments.