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

Abstract

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 of an improved statistical χ 2 treatment. In the framework of two neutrino oscillations we conclude that the best fit to the data is obtained in the LMA region with parameters (Δm 2, tan 2θ)=(5.4×10 −5 eV 2,0.38) , ( χ 2 min/ n=0.81, 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 a maximum statistical significance as low as ∼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 0.63 at the best fit LMA solution, where the DN asymmetry is negligible (approximately 10 −3). In the LOW region the signal is in the range ∼0.6–0.7 at 90% confidence level while the asymmetry is ≃1–20%. As a consequence, the combined Borexino measurements of the total event rate with an error below ±5–10% and day–night total rate asymmetry with a precision comparable to the one of SuperKamiokande will have a strong chance of selecting or at least strongly favoring one of the Solar neutrino solutions provided by present data.