Measurement of the solar B8 neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

We report the measurement of ν-e elastic scattering from B8 solar neutrinos with 3 MeV energy threshold by the Borexino detector in Gran Sasso (Italy). The rate of solar neutrino-induced electron scattering events above this energy in Borexino is 0.22±0.04(stat)±0.01(syst)cpd/100t, which corresponds to Φ_B8^ES=2.4±0.4±0.1×10⁶cm^-2s^-1, in good agreement with measurements from SNO and SuperKamiokaNDE. Assuming the B8 neutrino flux predicted by the high metallicity standard solar model, the average B8 ν_e survival probability above 3 MeV is measured to be 0.29±0.10. The survival probabilities for Be7 and B8 neutrinos as measured by Borexino differ by 1.9σ. These results are consistent with the prediction of the MSW-LMA solution of a transition in the solar ν_e survival probability P_ee between the low-energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.