2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/49241It is shown that only two four-neutrino schemes are compatible with the results of all neutrino oscillation experiments. These two schemes have a mass spectrum composed of two pairs of neutrinos with close masses separated by the "LSND gap" of the order of 1 eV.8 pages. Talk presented at the Xth International School "Particles and Cosmology", 19-25 April 1999, Baksan Valley, Kabardino-Balkaria, RussiaHigh Energy Physics - PhenomenologyHigh Energy Physics - Experimenttext