Solar-neutrino solutions with matter-enhanced flavor-changing neutral-current scattering

We explore quantitatively recent suggestions that the solar-neutrino puzzle may be explained by matter enhancement of nonstandard neutral-current scattering of neutrinos on $d$ quarks. These neutral currents introduce two new parameters, relating to flavor-diagonal and off-diagonal $\nu -d$ scattering, which affect the dynamical mixing of two neutrino flavors in the Sun. We consider this mixing in three alternative scenarios: (a) massless neutrinos, (b) massive neutrinos with off-diagonal but no new diagonal currents, and (c) massive neutrinos with both off-diagonal and new diagonal currents. We determine the regions in parameter space that give consistency between the standard solar model and existing solar-neutrino data from the Homestake and Kamiokande-II experiments, and give the corresponding predictions for future experiments with gallium and superfluid helium detectors. We also discuss the more general situation, with nonstandard neutral-current scattering from $u$ quarks and electrons as well as from $d$ quarks, and consider the effects of transmission through the Earth. Finally, we examine the bounds on nonstandard neutral-current neutrino scattering imposed by other experiments; some particular classes of solution are thereby excluded, but a variety of possibilities remains.