Study on pep and CNO solar neutrino interaction rates 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 3.28±0.56stat±0.26syst day-1(100ton)-1. Assuming the pep neutrino flux predicted by the Standard Solar Model, we obtained a constraint on the CNO solar neutrino interaction rate of <8.3 day-1(100 ton)-1 (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.99997% C.L., while the absence of the pep neutrino signal is disfavored at 98% C.L. The necessary sensitivity was achieved due to the extremely low levels of radioactive contamination in Borexino and by adopting data analysis techniques for the rejection of cosmogenic 11C, the dominant background in the 1–2 MeV region. Assuming the LMA-MSW solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.7±0.3)×108 cm-2s-1 and <7.9×108 cm-2s-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 [1].;[1] G. Bellini et al. First evidence of pep solar neutrinos by direct detection in Borexino. Phys.Rev.Lett., 108:051302, 2012.