Measurement of the solar<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">B</mml:mi><mml:mprescripts/><mml:none/><mml:mn>8</mml:mn></mml:mmultiscripts></mml:math>neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

Abstract

We report the measurement of $\ensuremath{\nu}\mathrm{\text{\ensuremath{-}}}e$ 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 $0.22\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.01(\mathrm{syst})\text{ }\text{ }\mathrm{cpd}/100\text{ }\text{ }\mathrm{t}$, which corresponds to ${\ensuremath{\Phi}}_{^{8}\mathrm{B}}^{\mathrm{ES}}=2.4\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.1\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, 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 $^{8}\mathrm{B}$ ${\ensuremath{\nu}}_{e}$ survival probability above 3 MeV is measured to be $0.29\ifmmode\pm\else\textpm\fi{}0.10$. The survival probabilities for $^{7}\mathrm{Be}$ and $^{8}\mathrm{B}$ neutrinos as measured by Borexino differ by $1.9\ensuremath{\sigma}$. These results are consistent with the prediction of the MSW-LMA solution of a transition in the solar ${\ensuremath{\nu}}_{e}$ survival probability ${P}_{ee}$ between the low-energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.