Measurement of the solar 8B neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

We report the measurement of {nu}-e elastic scattering from {sup 8}B 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/100 t, which corresponds to {Phi}{sub {sup 8}B}{sup ES}=2.4{+-}0.4{+-}0.1x10{sup 6} cm{sup -2} s{sup -1}, in good agreement with measurements from SNO and SuperKamiokaNDE. Assuming the {sup 8}B neutrino flux predicted by the high metallicity standard solar model, the average {sup 8}B {nu}{sub e} survival probability above 3 MeV is measured to be 0.29{+-}0.10. The survival probabilities for {sup 7}Be and {sup 8}B neutrinos as measured by Borexino differ by 1.9{sigma}. These results are consistent with the prediction of the MSW-LMA solution of a transition in the solar {nu}{sub e} survival probability P{sub ee} between the low-energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.