Status of the MSW solutions of the solar neutrino problem

We present an updated global analysis of two-flavour MSW solutions to the solar neutrino problem. 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. In our analysis we use all measured total event rates as well as all Super-Kamiokande data on the zenith angle dependence, energy spectrum and seasonal variation of the events. We compare the quality of the solutions of the solar neutrino anomaly in terms of conversions of v(e) into active or sterile neutrinos. For the case of 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 equally good Fit to the data. We find that the best-fit points for the combined analysis are Delta m(2) = 3.6 X 10(-5) eV(2) and sin(2)2 theta = 0.79 with chi(min)(2) = 35.4/30 d.o.f. and Delta m(2) = 5.1 x 10(-6) eV(2) and sin(2)2 theta = 5.5 X 10(-3) with chi(min)(2) = 37.4/30 dof 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. On the other hand, we show that seasonal effects, although small, may still leach 11% in the lower part of the LMA region, without conflict with the negative hints of a day-night variation (6% is due to the eccentricity of the Earth's orbit). In particular the best-fit LMA solution predicts a seasonal effect of 8.5%. For conversions into sterile neutrinos only the SMA solution is possible with best-fit point Delta m(2) = 5.0 X 10(-6) eV(2) and sin(2)2 theta = 3. X 10(-3) and chi(min)(2) = 40.2/30 d.o.f. We also consider departures of the Standard Solar Model (SSM) of Bahcall and Pinsonneault 1998 (BP98) by allowing arbitrary B-8 and hep fluxes. These modifications do not alter significantly the oscillation parameters. The best fit is obtained for B-8/B-8(SSM) = 0.61 and hep/hep(SSM) = 12 for the SMA solution both fur conversions into active or sterile neutrinos and B-8/B-8(SSM)=1.37 and hep/hep(SSM) = 38 for the LMA solution. (C) 2000 Elsevier Science B.V. All rights reserved.