Solar Neutrinos: Hint for Neutrino Mass

Abstract

The present status of the solar neutrino problem is reviewed. The strongest motivation for new neutrino physics comes from the complete phenomenological failure of astrophysical solutions: (1) The standard solar model is excluded by each of the solar neutrino experiments. (2) The combined results of Homestake and Kamiokande are incompatible with any astrophysical solution. (3) Even if the Homestake results were ignored entirely, there is no realistic solar model so far that describes the Kamiokande and gallium data simultaneously within their experimental uncertainties. On the other hand, the MSW solutions provide a complete description of the data, strongly hinting at the existence of mass and mixing of neutrinos. The MSW predictions for the future experiments are given in detail. It is stressed that solar model uncertainties are significant in determining the oscillation parameters and in making predictions for the future experiments. Especially, a $^8$B flux (or $S_{17}$) smaller by 20\% than that of the Bahcall-Pinsonneault model can make the interpretation of the charged to neutral current ratio measurement in SNO ambiguous.