Atmospheric and solar neutrino oscillations in νMSSM and beyond

We show how a unified description of the various two-flavor neutrino oscillation solutions, allowed by the atmospheric and solar neutrino experiments, are naturally realized within the framework of νMSSM (MSSM augmented with the seesaw mechanism) and beyond, especially grand unified theories. A general mechanism for achieving maximal mixing to resolve the atmospheric anomaly is discussed, and applied to the flipped SU(5) model. Except in the case of MSSM and SU(5), a light sterile neutrino is an inevitable consequence of our considerations. Neutrino hot dark matter is predicted in those models which exhibit bi-maximal neutrino mixings. A U(1) flavor symmetry, motivated by the charged fermion mass hierarchies and the magnitudes of the CKM matrix elements, plays a central role.