Measurement of the solar 8B neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

TL;DR

This paper reports the measurement of solar 8B neutrino flux using Borexino with a 3 MeV energy threshold, confirming neutrino oscillation models and providing precise flux and survival probability data.

Contribution

First measurement of 8B solar neutrino flux with a liquid scintillator detector at a 3 MeV threshold, supporting MSW-LMA oscillation predictions.

Findings

Measured 8B neutrino flux consistent with SNO and SuperKamiokaNDE.

Determined 8B neutrino survival probability to be 0.29+-0.10.

Observed a 1.9 sigma difference between 7Be and 8B neutrino survival probabilities.

Abstract

We report the measurement of electron neutrino elastic scattering from 8B solar neutrinos with 3 MeV energy threshold by the Borexino detector in Gran Sasso (Italy). The rate of solar neutrino-induced electron scattering events above this energy in Borexino is 0.217 +- 0.038 (stat) +- 0.008 (syst) cpd/100 t, which corresponds to the equivalent unoscillated flux of (2.4 +- 0.4 (stat) +- 0.1 (syst))x10^6 cm^-2 s^-1, in good agreement with measurements from SNO and SuperKamiokaNDE. Assuming the 8B neutrino flux predicted by the high metallicity Standard Solar Model, the average 8B neutrino survival probability above 3 MeV is measured to be 0.29+-0.10. The survival probabilities for 7Be and 8B neutrinos as measured by Borexino differ by 1.9 sigma. These results are consistent with the prediction of the MSW-LMA solution of a transition in the solar electron neutrino survival probability between the low energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.