Model-independent determination of the solar neutrino spectrum with and without the MSW effect

Aside from the opportunity for discovering new neutrino physics, solar neutrino measurements provide a sensitive probe of the solar interior, and thus a rigorous test of solar model predictions. We present model-independent determinations of the neutrino spectrum by using relevant flux components as free parameters subject only to the luminosity constraint. (1) Without the Mikheyev-Smirnov-Wolfenstein (MSW) effect, the best fit for the combined data is poor. Furthermore, the data indicate a severe suppression of the Be7 flux relative to the B8, contradicting both standard and nonstandard solar models in general; the pp flux takes its maximum value allowed by the luminosity constraint. This pathology consistently appears even if we ignore any one of the three data. (2) In the presence of the two-flavor MSW effect, the current constraint on the initial B8 fluxe is weak, but consistent with the SSM and sufficient to exclude nonstandard models with small B8 fluxes. No meaningful constraint is obtained for the other fluxes. In the future, even allowing the MSW effect, the B8 and Be7 fluxes can be determined at the ±(15-20)% level, making competing solar models distinguishable. We emphasize that the neutral current sensitivity for Be7 neutrinos in BOREXINO, HELLAZ, and HERON is essential for determining the initial fluxes. The constraints on the MSW parameters in the model-independent analysis are also discussed.