Matter-enhanced Three-flavor Oscillations and the Solar Neutrino Problem
  We present a systematic analysis of the three-flavor Mikheyev-Smirnov-Wolfenstein (MSW) oscillation solutions to the solar neutrino problem, in the hypothesis that the two independent neutrino square mass differences, $\delta m^2$ and $m^2$, are well separated: $\delta m^2 \ll m^2$. At zeroth order in $\delta m^2/m^2$, the relevant variables for solar neutrinos are $\delta m^2$ and two mixing angles, $\omega$ and $\phi$. We introduce new graphical representations of the parameter space $(\delta m^2,\,\omega,\,\phi)$, that prove useful both to analyze the properties of the electron-neutrino survival probability and to present the results of the analysis of solar neutrino data. We make a detailed comparison between the theoretical predictions of the Bahcall--Pinsonneault standard solar model and the current experimental results on solar neutrino rates, and discuss thoroughly the MSW solutions found by spanning the whole three-flavor space $(\delta m^2,\,\omega,\,\phi)$. The allowed regions can be radically different from the usual ``small mixing'' and ``large mixing'' solutions, characteristic of the usual two-generation MSW approach. We also discuss the link between these results and the independent information on neutrino masses and mixings coming from accelerator and reactor oscillation searches. 
G.L. Fogli (Bari U. & INFN, Bari), E. Lisi (Princeton, Inst. Advanced   Study & Bari U. & INFN, Bari), D. Montanino (Bari U....