Lepton mixing and neutrino oscillations

The present article is a review of phenomena connected with neutrino oscillations. Mixing of two neutrinos (Majorana as well as Dirac) with masses m1 and m2 is considered in detail. It is shown that the hypothesis of lepton mixing is not in contradiction with the existing data if mod(m12-m22)approximately0.01(eV)2. The possibilities of searching for oscillations by experiments on cosmic ray neutrinos and especially on solar neutrinos are discussed in detail. The last experiments have an incredible sensitivity from the point of view of testing the lepton mixing hypothesis (oscillation effects might be observable if mod(m12-m22)=2 neutrino types. In conclusion the case of heavy lepton mixing is considered. It is shown that in a concrete scheme with right-handed currents, the probabilities of such processes as μ→eγ, μ→3e etc. can be close to existing experimental upper limits, provided the heavy lepton masses are of an order of a few GeV, whereas the probabilities of the above processes are entirely negligible if only neutrinos are mixed. (Auth.)