Neutrino Magnetic Moments, Flavor Mixing, and the Super-Kamiokande Solar Data

We find that magnetic neutrino-electron scattering is unaffected by oscillations for vacuum mixing of Dirac neutrinos with only diagonal moments and for Majorana neutrinos with two flavors. For Mikheyev-Smirnov-Wolfenstein mixing, these cases are again obtained, though the effective moments can depend on the neutrino energy. Thus, e.g., the magnetic moments measured with ${\overline{\nu }}_e$ from a reactor and ${\nu }_e$ from the Sun could be different. With minimal assumptions, we find a new limit on ${\mu }_{\nu }$ using the 825-d Super-Kamiokande solar neutrino data: $|{\mu }_{\nu }|\le 1.5\times {10}^{-10\mathrm{}}{\mu }_B$ at $90\%$ CL, comparable to the existing reactor limit.