Measurement of the solarB8neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

Abstract

We report the measurement of $\nu \mathrm{\text{−}}e$ elastic scattering from ${}^8\mathrm{B}$ 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 $0.22\pm 0.04(\mathrm{stat})\pm 0.01(\mathrm{syst})\mathrm{cpd}/100\mathrm{t}$, which corresponds to ${\Phi }_{{}^8\mathrm{B}}^{\mathrm{ES}}=2.4\pm 0.4\pm 0.1\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, in good agreement with measurements from SNO and SuperKamiokaNDE. Assuming the ${}^8\mathrm{B}$ neutrino flux predicted by the high metallicity standard solar model, the average ${}^8\mathrm{B}$ ${\nu }_e$ survival probability above 3 MeV is measured to be $0.29\pm 0.10$. The survival probabilities for ${}^7\mathrm{Be}$ and ${}^8\mathrm{B}$ 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 ${\nu }_e$ survival probability $P_{ee}$ between the low-energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.