NEUTRINO MASSES AND MIXING IN THE LIGHT OF EXPERIMENTAL-DATA

All the possible schemes of neutrino mixing with four massive neutrino s inspired by the existing experimental indications in favour of neutr ino mixing are considered. It is shown that the scheme with a neutrino mass hierarchy is not compatible with the experimental results, likew ise all other schemes with the masses of three neutrinos close togethe r and the fourth mass separated by a gap needed to incorporate the LSN D neutrino oscillations. Only two schemes with two pairs of neutrinos with close masses separated by this gap of the order of 1 eV are in ag reement with the results of all experiments. We carefully examine the arguments leading to this conclusion and also discuss experimental con sequences of the two favoured neutrino schemes.