On the neutrino mass spectrum and neutrino mixing from oscillation data

Two schemes of mixing of four massive neutrinos with two couples of close neutrino masses separated by a gap of the order of 1 eV can accommodate solar, atmospheric and LSND neutrino oscillation data. It is shown that long-baseline /-bare → /-bare and νμ → νe transitions are strongly suppressed in these schemes. The scheme of mixing of three neutrino masses with a mass hierarchy that can describe solar and atmospheric neutrino data is also discussed. It is shown that in this scheme the effective Majorana mass veretical bar vertical bar that characterizes the matrix element of neutrinoless double-β decay is less than ∼ 10-2 eV