Status of Neutrino Mixing∗

The investigation of the problem of neutrino masses and mixing is the central theme of today’s neutrino physics. This problem is very important for elementary particle physics. There is a general belief that its investigation is a way to discover new physics. The problem of the masses of the neutrinos has also an exceptional importance for astrophysics. If the neutrino masses are in the eV region, it will allow to solve the problem (or part of the problem) of dark matter. After many years of investigations, the problem of neutrino masses and mixing is still far from being solved. At present we have several indications in favour of nonzero neutrino masses and mixing angle. These indications come first of all from the solar neutrino experiments (Homestake [1], Kamiokande [2], GALLEX [3] and SAGE [4]). There are also indications in favour of neutrino mixing from some experiments on the detection of atmospheric neutrinos [5, 6, 7] and from the LSND experiment [8] in which beam-stop neutrinos were detected. In the future with new experiments Super-Kamiokande [9], SNO [10], CHORUS [11], NOMAD [12], CHOOZ [13], ICARUS [14], MINOS [15], COSMOS [16] and many others we can expect a real progress in the investigation of the problem of neutrino masses, neutrino mixing and neutrino nature. I will start with the formulation of the hypothesis of neutrino mixing. In accordance with all existing experimental data, the interaction of neutrinos with matter is described by the standard CC and NC Lagrangians