First evidence of pep solar neutrinos by direct detection in Borexino

TL;DR

This paper reports the first direct detection of pep solar neutrinos in Borexino, providing new measurements of solar neutrino fluxes and constraining the CNO neutrino flux with unprecedented sensitivity using novel background rejection techniques.

Contribution

First direct detection of pep solar neutrinos in Borexino, with improved sensitivity and novel data analysis methods for background rejection.

Findings

Measured pep neutrino interaction rate: 3.1±0.6(stat)±0.3(syst) counts/(day×100 ton)

Set an upper limit on CNO neutrino interaction rate: <7.9 counts/(day×100 ton) (95% C.L.)

Confirmed solar neutrino fluxes in agreement with the Standard Solar Model

Abstract

We observed, for the first time, solar neutrinos in the 1.0-1.5 MeV energy range. We measured the rate of pep solar neutrino interactions in Borexino to be [3.1+-0.6(stat)+-0.3(syst)] counts/(day x 100 ton) and provided a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day x 100 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. This unprecedented sensitivity was achieved by adopting novel data analysis techniques for the rejection of cosmogenic 11C, the dominant background in the 1-2 MeV region. Assuming the MSW-LMA solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of [1.6+-0.3]x10^8 cm^-2s-1 and 7.7x10^8 cm^-2s-1 (95% C.L.), respectively, in agreement with the Standard Solar Model. These results represent the first measurement of the pep neutrino flux and the strongest constraint of the CNO solar neutrino flux to date.