Direct Measurement of the ${}^7\mathrm{Be}$ Solar 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.

Figure 1

The raw photoelectron charge spectrum after the basic cuts (i)–(iii) (black), after the fiducial cut (iv) (blue), and after the statistical subtraction of the $\alpha$-emitting contaminants (red). All curves scaled to the exposure of $100\mathrm{day}·\mathrm{ton}$. Cuts described in the text.

Figure 2

Spectral fit in the energy region 160–2000 keV. Contributions from ${}^{214}\mathrm{Pb}$, $pp$, and $pep$ neutrinos, not shown, are almost negligible with respect to those in the figure.

Figure 3

Spectral fit in the energy region 260–1670 keV prior to statistical $\alpha$’s subtraction.

Figure 4

Determination of flux normalization constants for $pp$ and CNO solar neutrinos, $f_{pp}$ and $f_{\mathrm{CNO}}$ (68%, 90%, and 99% C.L.).