Mikheyev-Smirnov-Wolfenstein enhancement of oscillations as a possible solution to the solar-neutrino problem

Mikheyev and Smirnov have observed that neutrino oscillations in the Sun can be greatly enhanced through the mechanism of Wolfenstein matter oscillations. We develop a qualitative understanding of this phenomenon in the small-mixing-angle limit and carry out extensive calculations in order to apply it to the solar-neutrino problem. Our simple theoretical model agrees remarkably well with the calculations. After determining those values of m^2 and sin^22theta in the small-mixing-angle limit for which the ^8B plus ^7Be neutrino capture rate in ^37Cl is suppressed by a factor 2--4, we predict the corresponding capture rate for pp plus ^7Be neutrinos in ^71Ga. The gallium capture rate can range from no reduction to a factor of 10 reduction. We also determine the modified spectrum of ^8B neutrinos arriving at Earth and discuss the importance of this spectrum as a means of choosing between oscillations and the solar model as the cause of the solar-neutrino problem, and also as a means of distinguishing between different sets of oscillation parameters.