Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino

The rate of neutrino-electron elastic scattering interactions from 862 keV 7Be solar neutrinos in Borexino is determined to be 46.0±1.5(stat)-1.6+1.5(syst) counts/(day·100 ton). This corresponds to a νe-equivalent 7Be solar neutrino flux of (3.10±0.15)x109 cm-2 s-1 and, under the assumption of νe transition to other active neutrino flavours, yields an electron neutrino survival probability of 0.51±0.07 at 862 keV. The no flavor change hypothesis is ruled out at 5.0 σ. A global solar neutrino analysis with free fluxes determines Φpp=6.06-0.06+0.02x1010 cm-2 s-1 and ΦCNO<1.3x109 cm-2 s-1 (95% C.L.). These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.