Measurement of the solar 8B neutrino flux down to 2.8 MeV with Borexino

We report the measurement of the 8 B solar neutrinos interaction rate with the Borexino detector. The extremly high radio-purity reached in the Borexino scintillator, combined with the efficient software rejection of cosmogenic background, allows to investigate the recoiled electron spectrum, induced by 8 B solar neutrinos, down to the unprecedented energy threshold of 2.8 MeV. The rate of 8 B solar neutrino interaction as measured through their scattering on the target electrons is 0.26 ± 0.04 stat ± 0.02 syst c/d/100 tons. This corresponds to an equivalent electron neutrino flux of ( 2.65 ± 0.44 stat ± 0.18 syst ) × 10 6 cm − 2 s − 1 , as derived from the elastic scattering only, in good agreement with existing measurements and predictions.