Neutrino masses: from fantasy to facts

Abstract

Theory suggests the existence of neutrino masses, but little more. Facts are coming close to revealing our fantasy: solar- and atmospheric-neutrino data strongly indicate the need for neutrino conversions, while LSND provides an intriguing hint. The simplest ways to reconcile these data in terms of neutrino oscillations invoke a light sterile neutrino in addition to the three active ones. Out of the four neutrinos, two are maximally mixed and lie at the LSND scale, while the others are at the solar-mass scale. These schemes can be distinguished at neutral-current-sensitive solar- and atmospheric-neutrino experiments. I discuss the simplest theoretical scenarios, where the lightness of the sterile neutrino, the nearly maximal atmospheric-neutrino mixing and the generation of Δm ⊙ 2 and Δm atm 2 all follow naturally from the assumed lepton-number symmetry and its breaking. Although the most likely interpretation of the present data is in terms of neutrino-mass-induced oscillations, one still has room for alternative explanations, such as flavor-changing neutrino interactions, with no need for neutrino mass or mixing. Such flavor-violating transitions arise in theories with strictly massless neutrinos and may lead to other sizeable flavor non-conservation effects, such as μ → e + γ, μ − e conversion in nuclei, unaccompanied by neutrinoless double-beta decay.