Solar neutrino experiments: results and implications

The neutrinos that are produced in the solar fusion reaction chains reflect the conditions in the stellar interior. If the model predictions for the expected neutrino fluxes are firm, then the solar neutrino experiments can also test neutrino properties during propagation along the $1.5\times {10}^8\mathrm{kilometer}$ “baseline” between “source” and detector. So far, results have been acquired with five solar neutrino detectors: Homestake, Kamiokande, Superkamiokande, Gallex, and Sage. Taking all of the experimental data together, it appears that the abundant pp neutrinos are present, the moderately abundant ${}^7\mathrm{Be}$ neutrinos are strongly or totally suppressed, and the rare ${}^8\mathrm{B}$ neutrinos are partially reduced, by a factor of about three. This outcome cannot be explained by modifications of the stellar model but it is consistent with the hypothesis of neutrino flavor oscillations. Such oscillations could occur on the way between the solar core and the detector. In such a scenario, a nonvanishing neutrino rest mass would follow. The evidence is summarized here, and the outlook as regards anticipated future experiments is briefly discussed.