Neutrino oscillations from Dirac and Majorana masses

We present a scenario of neutrino masses and mixing angles. Each generation includes a sterile right-handed neutrino in addition to the usual left-handed one. We assume a hierarchy in their Dirac masses similar to, but much larger than, the hierarchies in the quarks and charged leptons. In addition, we include a Majorana mass term for the sterile neutrinos only. These assumptions prove sufficient to accommodate scales of mass differences and mixing angles consistent with all existing neutrino oscillation data. copyright 1997 The American Physical Society