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

We report the measurement of the 8B solar neutrinos interaction rate with the Borexino detector. The extremely 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 8B solar neutrinos, down to the unprecedented energy threshold of 2.8 MeV. The rate of 8B solar neutrino interaction as measured through their scattering on the target electrons is 0.26±0.04stat±0.02syst c/d/100 tons. This corresponds to an equivalent electron neutrino flux of (2.65±0.44stat±0.18syst)x106 cm-2s-1, as derived from the elastic scattering only, in good agreement with existing measurements and predictions.