Natural framework for solar and 17-keV neutrinos

Motivated by recent experimental claims for the existence of a 17-keV neutrino and by the solar-neutrino problem, we consider a class of models which contain in their low-energy spectrum a single light sterile neutrino and one or more Nambu-Goldstone bosons. In these models, the required pattern of small neutrino masses and Nambu-Goldstone-boson couplings are understood as the low-energy residue of the pattern of breaking of lepton-number symmetries near the electroweak scale, and all mass hierarchies are technically natural. The models are compatible with all cosmological and astrophysical constraints, and can solve the solar-neutrino problem either via the Mikheyev-Smirnov-Wolfenstein effect or vacuum oscillations. The deficit in atmospheric muon neutrinos seen in the Kamiokande and IMB detectors can also be explained in these models.