The phenomenon of neutrino oscillations is reviewed. The case of mixing of two neutrinos (both Majorana and Dirac neutrinos) of masses $m_1$ and $m_2$ is considered in detail. It is shown that the mixing hypothesis is consistent with the existing data if $\vert m_1^2-m_2^2\vert\lesssim$ 1eV$^2$ (for maximal mixing). A discussion is given of possible experiments to search for oscillations of neutrinos from reactors, meson factories, and highenergy accelerators. Oscillations can be observed in these experiments if $\vert m_1^2-m_2^2\vert\gtrsim10^{-2}$eV$^2$. The prospects of searches for neutrino oscillations in experiments using cosmic neutrinos, and in particular solar neutrinos, are discussed in detail. Such experiments have a unique sensitivity as tests of the mixing hypothesis (in the case of solar neutrinos, mixing effects would be observed if $\vert m_1^2-m_2^2\vert\gtrsim10^{-12}$eV$^2$. The ``solar neutrino paradox'' is discussed in detail from the point of view of neutrino mixing. Neutrino oscillations are also considered in the case when there exist in nature $N$>2 types of neutrinos. Schemes involving the mixing of heavy leptons are discussed. It is shown in the framework of a specific scheme involving right-handed currents that for heavy-lepton masses of the order of several GeV the probabilities of processes such as $\mu\to e\gamma$ and $\mu\to3e$ can be close to the existing experimental upper limits. The probabilities of these processes are practically zero when only the neutrinos are mixed.

RT Journal
T1 Lepton mixing and neutrino oscillations
A1 Bilen’kii,S.M.
A1 Pontecorvo,B.
PB Physics-Uspekhi
PY 1977
FD 10 Oct, 1977
JF Physics-Uspekhi
JO Phys. Usp.
VO 20
IS 10
SP 776-795
DO 10.1070/PU1977v020n10ABEH005465
LK https://ufn.ru/en/articles/1977/10/b/