Neutrino Masses, where do we stand?

Abstract

I review the status of neutrino physics post-Neutrino~98, including the implications of solar and atmospheric neutrino data, which strongly indicate nonzero neutrino masses. LSND and the possible role of neutrinos as hot dark matter (HDM) are also mentioned. The simplest schemes proposed to reconcile these requirements invoke a light sterile neutrino in addition to the three active ones, two of them at the MSW scale and the other two maximally-mixed neutrinos at the HDM/LSND scale. In the simplest theory the latter scale arises at one-loop, while the solar and atmospheric parameters $Δ{m^2}_\odot$ & $Δ{m^2}_{atm}$ appear at the two-loop level. The lightness of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and the generation of $Δ{m^2}_\odot$ & $Δ{m^2}_{atm}$ follow naturally from the assumed lepton-number symmetry and its breaking. These two basic schemes can be distinguished at future solar & atmospheric neutrino experiments and have different cosmological implications.