New results on solar neutrino fluxes from 192 days of Borexino data

Abstract

We report the direct measurement of the ^7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV ^7Be neutrinos is 49+-3(stat)+-4(syst) counts/(day * 100ton). The hypothesis of no oscillation for ^7Be solar neutrinos is inconsistent with our measurement at the 4sigma level. 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 ^7Be, pp, and CNO solar nu_e, and the limit on the magnetic moment of neutrinos.

Figures (4)

• 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 day$\cdot$ton. Cuts described in the text.
• Figure 2: Spectral fit in the energy region 160--2000 keV.
• 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 $f_{\rm CNO}$ (68%, 90%, and 99% C.L.).