Rescuing grand unification scenario for neutrino mass

The see-saw mechanism for neutrino masses based on the Grand Unification leads to the mass of the heaviest neutrino (≈ντ) in the range (2−3)·10−3 eV and hence to a solution of the solar neutrino problem through the νe→ντ conversion. We suggest the existence of a light singlet fermion S which mixes predominantly with muon neutrino through the mixing mass mμs∼O(1) eV. The introduction of such a singlet allows one (i) to solve the atmospheric neutrino problem via the νμ↔S oscillations, (ii) to explain the LSND result and (iii) to get two component hot dark matter in the Universe. The phenomenology of this scenario is considered and the origin of the singlet as well as of the scale mμs is discussed.