Precision Measurement of the Be7 Solar Neutrino Interaction Rate in Borexino

The rate of neutrino-electron elastic scattering interactions from 862 keV $^{7}mathrm{Be}$ solar neutrinos in Borexino is determined to be $46.0ifmmodepmelsetextpmfi{}1.5(mathrm{stat}{)}_{ensuremath{-}1.6}^{+1.5}(mathrm{syst})text{ }mathrm{text{counts}}/(mathrm{day}ifmmodecdotelsetextperiodcenteredfi{}100text{ }text{ }mathrm{ton})$. This corresponds to a ${ensuremath{nu}}_{e}$-equivalent $^{7}mathrm{Be}$ solar neutrino flux of $(3.10ifmmodepmelsetextpmfi{}0.15)ifmmodetimeselsetexttimesfi{}{10}^{9}text{ }text{ }{mathrm{cm}}^{ensuremath{-}2}text{ }{mathrm{s}}^{ensuremath{-}1}$ and, under the assumption of ${ensuremath{nu}}_{e}$ transition to other active neutrino flavours, yields an electron neutrino survival probability of $0.51ifmmodepmelsetextpmfi{}0.07$ at 862 keV. The no flavor change hypothesis is ruled out at $5.0text{ }ensuremath{sigma}$. A global solar neutrino analysis with free fluxes determines ${ensuremath{Phi}}_{pp}={6.06}_{ensuremath{-}0.06}^{+0.02}ifmmodetimeselsetexttimesfi{}{10}^{10}text{ }text{ }{mathrm{cm}}^{ensuremath{-}2}text{ }{mathrm{s}}^{ensuremath{-}1}$ and ${ensuremath{Phi}}_{mathrm{CNO}}<1.3ifmmodetimeselsetexttimesfi{}{10}^{9}text{ }text{ }{mathrm{cm}}^{ensuremath{-}2}text{ }{mathrm{s}}^{ensuremath{-}1}$ (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.