Direct Measurement of theBe7Solar Neutrino Flux with 192 Days of Borexino Data

We report the direct measurement of the ${}^7\mathrm{Be}$ solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV ${}^7\mathrm{Be}$ neutrinos is $49\pm 3_{\mathrm{stat}}\pm 4_{\mathrm{syst}}\mathrm{\text{counts}}/(\mathrm{day}·100\mathrm{ton})$. The hypothesis of no oscillation for ${}^7\mathrm{Be}$ solar neutrinos is inconsistent with our measurement at the $4\sigma$ C.L. Our result is the first direct measurement of the survival probability for solar ${\nu }_e$ in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of ${}^7\mathrm{Be}$, $pp$, and CNO solar ${\nu }_e$, and the limit on the effective neutrino magnetic moment using solar neutrinos.