Direct Measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi>Be</mml:mi><mml:mprescripts/><mml:none/><mml:mn>7</mml:mn></mml:mmultiscripts></mml:math>Solar Neutrino Flux with 192 Days of Borexino Data

Abstract

We report the direct measurement of the $^{7}\mathrm{Be}$ solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV $^{7}\mathrm{Be}$ neutrinos is $49\ifmmode\pm\else\textpm\fi{}{3}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{4}_{\mathrm{syst}}\text{ }\text{ }\mathrm{\text{counts}}/(\mathrm{day}\ifmmode\cdot\else\textperiodcentered\fi{}100\text{ }\text{ }\mathrm{ton})$. The hypothesis of no oscillation for $^{7}\mathrm{Be}$ solar neutrinos is inconsistent with our measurement at the $4\ensuremath{\sigma}$ C.L. Our result is the first direct measurement of the survival probability for solar ${\ensuremath{\nu}}_{e}$ in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of $^{7}\mathrm{Be}$, $pp$, and CNO solar ${\ensuremath{\nu}}_{e}$, and the limit on the effective neutrino magnetic moment using solar neutrinos.