Neutrino magnetic moments, flavor mixing, and the SuperKamiokande 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 MSW mixing, these cases again obtain, though the effective moments can depend on the neutrino energy. Thus, e.g., the magnetic moments measured with $\bar{\nu}_e$ 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 SuperKamiokande solar neutrino data: $\mu_{\nu} \le 2.0\times 10^{-10} \mu_B$ with the published 504-days data, and $\mu_{\nu} \le 1.6 \times 10^{-10} \mu_B$ with the preliminary 708-days data (with two new low-energy bins), both at 90% CL.