Status of the MSW Solutions to the Solar Neutrino Problem

Abstract

We present an updated global analysis of two-flavor MSW solutions to the solar neutrino problem in terms of conversions of nu_e into active or sterile neutrinos. We perform a fit to the full data set corresponding to the 825-day Super-Kamiokande data sample as well as to Chlorine, GALLEX and SAGE experiments. We use all measured total event rates as well as Super-Kamiokande data on the zenith angle dependence, energy spectrum and seasonal variation of the events. For conversions into active neutrinos we find that, although the data on the total event rates favours the Small Mixing Angle (SMA) solution, once the full data set is included both SMA and Large Mixing Angle (LMA) solutions give an equivalent fit to the data. The best-fit points for the combined analysis are Delta m^2=3.6 10^{-5} eV^2 sin^2(2theta)=0.79 with chi^2_min=35.4/30 d.o.f and Delta m^2=5.1~ 10^{-6} eV^2 and sin^2(2theta)=5.5 10^{-3} with chi^2_min=37.4/30 d.o.f. In contrast with the earlier 504-day study of Bahcall-Krastev-Smirnov our results indicate that the LMA solution is not only allowed, but slightly preferred. We also show that seasonal effects, although small, may still reach 8.5 % at the best-fit LMA solution without conflict with the hints of a day-night variation (6% is due to the eccentricity of the Earth's orbit). For conversions into sterile neutrinos only the SMA solution is allowed with best-fit point Delta m^2=5.0 10^{-6} eV^2 and sin^2(2theta)=3. 10^{-3} and chi^2_min=40.2/30 d.o.f. We also consider departures of the Standard Solar Model of Bahcall and Pinsonneault 1998 by allowing arbitrary 8^B and hep fluxes. Best fit is obtained for ^8B/^8B_SSM=0.61 (1.37) and hep/hep_SSM=12 (38) for the SMA (LMA) solution.