Lepton Mixing: Small, Large, Maximal?

Abstract

The SuperKamiokande data on atmospheric neutrinos imply that $\nu_{\mu}$ has large (or even maximal) mixing. It is still open question whether this mixing is the flavor one or mixing with singlet state (sterile neutrino). Several tests exist to establish the channel of atmospheric neutrino oscillations. Large mixing can be a property of the second and third generations, so that the scheme with single large mixing is realized. It can be a generic property of all leptons, thus leading to the bi-large or threefold large mixing schemes. The ambiguity will be resolved by identification of solution of the solar neutrino problem. In this review we consider phenomenology of various mixing schemes. The key measurements which will allow us to make significant step in reconstruction of the neutrino mass and flavor spectrum are discussed.