Three-flavor MSW solutions of the solar neutrino problem

We perform an updated phenomenological analysis of the Mikheyev-Smirnov-Wolfenstein (MSW) solutions of the solar neutrino problem, assuming oscillations between two and three neutrino families. The analysis includes the total rates of the Homestake, SAGE, GALLEX, Kamiokande and Super-Kamiokande experiments, as well as the day-night asymmetry and the 18-bin energy spectrum of Super-Kamiokande. Solutions are found at several values of the θ₁₃ mixing angle. Among the most interesting features, we find that solar neutrino data alone put the constraint θ₁₃55^∘--59^∘ at 95% C.L., and that a fraction of the MSW solutions extends at and beyond maximal (ν₁,ν₂) mixing (θ₁₂>~π/4), especially if the neutrino square mass splitting is in its lower range (m₂²-m₁²~10^-7 eV²) and if θ₁₃ is nonzero. In particular, bimaximal (or nearly bimaximal) mixing is possible for atmospheric and MSW solar neutrino oscillations within the stringent reactor bounds on θ₁₃.