First Evidence ofpepSolar Neutrinos by Direct Detection in Borexino
We observed, for the first time, solar neutrinos in the 1.0--1.5 MeV energy range. We determined the rate of $pep$ solar neutrino interactions in Borexino to be $3.1\ifmmode\pm\else\textpm\fi{}{0.6}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.3}_{\mathrm{syst}}\text{ }\text{ }\mathrm{counts}/(\mathrm{day}\ifmmode\cdot\else\textperiodcentered\fi{}100\text{ }\text{ }\mathrm{ton})$. Assuming the $pep$ neutrino flux predicted by the standard solar model, we obtained a constraint on the CNO solar neutrino interaction rate of $<7.9\text{ }\text{ }\mathrm{counts}/(\mathrm{day}\ifmmode\cdot\else\textperiodcentered\fi{}100\text{ }\text{ }\mathrm{ton})$ ($95%$ C.L.). The absence of the solar neutrino signal is disfavored at $99.97%$ C.L., while the absence of the $pep$ signal is disfavored at $98%$ C.L. The necessary sensitivity was achieved by adopting data analysis techniques for the rejection of cosmogenic $^{11}\mathrm{C}$, the dominant background in the 1--2 MeV region. Assuming the Mikheyev-Smirnov-Wolfenstein large mixing angle solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of $(1.6\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ and $<7.7\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ ($95%$ C.L.), respectively, in agreement with both the high and low metallicity standard solar models. These results represent the first direct evidence of the $pep$ neutrino signal and the strongest constraint of the CNO solar neutrino flux to date.