The Simplest Resonant Spin--Flavour Solution to the Solar Neutrino Problem

We re-analyse the resonant spin-flavour (RSF) solutions to the solar neutrino problem in the framework of analytic solutions to the solar magneto-hydrodynamics (MHD) equations. By substantially eliminating the arbitrariness associated to the magnetic field profile due to both mathematical consistency and physical requirements we propose the simplest scheme (MHD-RSF, for short) for solar neutrino conversion using realistic static MHD solutions. Using such effective two-parameter scheme we perform the first global fit of the recent solar neutrino data, including event rates as well as zenith angle distributions and recoil electron spectra induced by solar neutrino interactions in Superkamiokande. We compare quantitatively our simplest MHD-RSF fit with vacuum oscillation (VAC) and MSW-type (SMA, LMA and LOW) solutions to the solar neutrino problem using a common well-calibrated theoretical calculation and fit procedure. We find our MHD-RSF fit to be somewhat better than those obtained for the favored neutrino oscillation solutions, though not in a statistically significant way. We briefly discuss the prospects to disentangle our MHD-RSF scenario at future solar neutrino experiments, giving some predictions for the SNO experiment.Comment: 26 pages with 6 postscript figures included. Improved presentation and misprints corrected. Final version to appear in Nucl. Phys.

Global analysis of Solar neutrino oscillation evidence including SNO and implications for Borexino

An updated analysis of all available neutrino oscillation evidence in Solar experiments including the latest $SNO$ data is presented. Predictions for total rates and day-night asymmetry in Borexino are calculated. Our analysis features the use of exhaustive computation of the neutrino oscillation probabilities and the use of an improved statistical $\chi^2$ minimization. In the framework of two neutrino oscillations we conclude that the best fit to the data is obtained in the LMA region with parameters (\Delta m^2, \tan^2\theta) = (5.2 \times 10^{-5} \eV^2, 0.47), ($\chi^2_{min}/n=0.82$, $n=38$ degrees of freedom). Although less favored, solutions in the LOW and VAC regions are still possible with a reasonable statistical significance. The best possible solution in the SMA region gets as maximum a statistical significance as low as $\sim 3$. We study the implications of these results for the prospects of Borexino and the possibility of discriminating between the different solutions. The expected normalized Borexino signal is 0.62 at the best fit LMA solution while the DN asymmetry is negligible (approximately $10^{-5}$). In the LOW region the signal is in the range $\sim 0.6-0.7$ at 90% confidence level while the asymmetry is $\simeq 1-20$. As a consequence, the combined Borexino measurements of the total event rate with a error below $\pm 5-10$ and day-night total rate asymmetry with a precision comparable to the one of SuperKamiokande, will have a strong chance of distinguishing or at least strongly favoring one of the Solar neutrino solutions provided by present data.Comment: 24 pages, 9 figures, better resolution figures are available upon reques

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