Measurement of the solar ${}^8\mathrm{B}$ neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

We report the measurement of $\nu \mathrm{\text{−}}e$ elastic scattering from ${}^8\mathrm{B}$ solar neutrinos with 3 MeV energy threshold by the Borexino detector in Gran Sasso (Italy). The rate of solar neutrino-induced electron scattering events above this energy in Borexino is $0.22\pm 0.04(\mathrm{stat})\pm 0.01(\mathrm{syst})\mathrm{cpd}/100\mathrm{t}$, which corresponds to ${\Phi }_{{}^8\mathrm{B}}^{\mathrm{ES}}=2.4\pm 0.4\pm 0.1\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, in good agreement with measurements from SNO and SuperKamiokaNDE. Assuming the ${}^8\mathrm{B}$ neutrino flux predicted by the high metallicity standard solar model, the average ${}^8\mathrm{B}$ ${\nu }_e$ survival probability above 3 MeV is measured to be $0.29\pm 0.10$. The survival probabilities for ${}^7\mathrm{Be}$ and ${}^8\mathrm{B}$ neutrinos as measured by Borexino differ by $1.9\sigma$. These results are consistent with the prediction of the MSW-LMA solution of a transition in the solar ${\nu }_e$ survival probability $P_{ee}$ between the low-energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.

Figure 1

The black dots are the measured peak positions of $\gamma$ radiation induced by neutron captures in ${}^1\mathrm{H}$ (2.223 MeV), ${}^{12}\mathrm{C}$ (4.945 MeV), ${}^{56}\mathrm{Fe}$ (7.631 MeV), and ${}^{54}\mathrm{Fe}$ (9.298 MeV) in the detector center. The red line is the Monte Carlo prediction for $\gamma$ rays generated in the detector center.

Figure 2

Ratio of the reconstructed radial position of $\gamma$ events from the ${}^{241}\mathrm{Am}{}^9\mathrm{Be}$ source in Borexino to the source radial position measured by the CCD camera system, as a function of the measured charge.

Figure 3

Cumulative distribution of events with energy $>3\mathrm{MeV}$ within a 5 s window after a muon in Borexino. The time distribution has been fit to three decay exponentials. The ensuing exponential lifetimes are $\tau =0.031\pm 0.002\mathrm{s}$, $0.25\pm 0.21\mathrm{s}$, $1.01\pm 0.36\mathrm{s}$ and corresponds to the contribution from ${}^{12}\mathrm{B}$, ${}^8\mathrm{He}+{}^9\mathrm{C}+{}^9\mathrm{Li}$, and ${}^8\mathrm{B}+{}^6\mathrm{He}+{}^8\mathrm{Li}$, respectively. The expected and measured rates for these cosmogenic isotopes are summarized in Table 1.

Figure 4

Time profile of events with energy $>3\mathrm{MeV}$ within 240 s after a muon and within 2 m from its track in the entire Borexino active volume (278 t). The distribution has been fit to the three decay exponentials as in Fig. 3, plus the ${}^{11}\mathrm{Be}$ component, with fixed mean lives.

Figure 5

Fit of the radial distribution for events with $E>3\mathrm{MeV}$. The line colors represent, from dark to light grey, radial distribution of external background, surface contamination, and the uniform distribution of the event component in the active mass

Figure 6

Comparison of the final spectrum after data selection (red dots) to Monte Carlo simulations (black line). The expected electron recoil spectrum from oscillated ${}^8\mathrm{B}$ $\nu$ interactions (filled blue histogram), ${}^{208}\mathrm{Tl}$ (green), ${}^{11}\mathrm{Be}$ (cyan), and external background (violet), are equal to the measured values in Table 3.

Figure 7

Comparison of the final spectrum after data selection and background subtraction (dots) to Monte Carlo simulations of oscillated ${}^8\mathrm{B}$ interactions, with amplitude from the standard solar models BPS09(GS98) (high metallicity) and BPS09(AGS05) (low metallicity), and from the MSW-LMA neutrino oscillation model.

Figure 8

Electron neutrino survival probability as a function of the neutrino energy, evaluated for the ${}^8\mathrm{B}$ neutrino source assuming the BPS09(GS98) standard solar model [11, 12, 13] and the oscillation parameters from the MSW-LMA solution $\Delta {\mathrm{m}}^2=7.69\times {10}^{-5}{\mathrm{eV}}^2$ and ${tan}^2\theta =0.45$ [24]). The dots represent the Borexino results from ${}^7\mathrm{Be}$ and ${}^8\mathrm{B}$ measurements. The error bars include also the theoretical uncertainty of the expected flux from the standard solar model BPS09(GS98).