Four-Neutrino Oscillations

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It is shown that at least four massive neutrinos are needed in order to accommodate the evidences in favor of neutrino oscillations found in solar and atmospheric neutrino experiments and in the LSND experiment. Among all four-neutrino schemes, only two, with a mass spectrum composed of two pairs of neutrinos with close masses separated by the "LSND gap" of the order of 1 eV, are compatible with the results of all neutrino oscillation experiments. In these two schemes the probability of nuₑ transitions into other states, the probability of nuₘu->nuₑ transitions and the size of CP violation effects in nuₘu->nuₑ transitions are suppressed in long-baseline experiments.