Precision Measurement of the ${}^7\mathrm{Be}$ 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 $46.0\pm 1.5(\mathrm{stat}{)}_{-1.6}^{+1.5}(\mathrm{syst})\mathrm{\text{counts}}/(\mathrm{day}·100\mathrm{ton})$. This corresponds to a ${\nu }_e$-equivalent ${}^7\mathrm{Be}$ solar neutrino flux of $(3.10\pm 0.15)\times {10}^9{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and, under the assumption of ${\nu }_e$ transition to other active neutrino flavours, yields an electron neutrino survival probability of $0.51\pm 0.07$ at 862 keV. The no flavor change hypothesis is ruled out at $5.0\sigma$. A global solar neutrino analysis with free fluxes determines ${\Phi }_{pp}={6.06}_{-0.06}^{+0.02}\times {10}^{10}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and ${\Phi }_{\mathrm{CNO}}<1.3\times {10}^9{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ (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.

Figure 1

Two example fitted spectra; the fit results in the legends have units [$\mathrm{\text{counts}}/(\mathrm{day}·100\mathrm{ton})$]. Top: A Monte Carlo based fit over the energy region 270–1600 keV to a spectrum from which some, but not all, of the $\alpha$ events have been removed using a PSA cut, and in which the event energies were estimated using the number of photons detected by the PMT array. Bottom: An analytic fit over the 290–1270 keV energy region to a spectrum obtained with statistical $\alpha$ subtraction and in which the event energies were estimated using the total charge collected by the PMT array. In all cases the fitted event rates refer to the total rate of each species, independent of the fit energy window.

Figure 2

The global experimental constraints on the low energy solar $P_{ee}$. For the ${}^7\mathrm{Be}$ point, which shows the current result, the inner (red) error bars show the experimental uncertainty, while the outer (blue) error bars show the total ($\mathrm{\text{experimental}}+\mathrm{SSM}$) uncertainty. The remaining points were obtained following the procedure in [22], wherein the survival probabilities of the low energy ($pp$), medium energy, and high energy (${}^8\mathrm{B}$) solar neutrinos are obtained, with minimal model dependence, from a combined analysis of the results of all solar neutrino experiments. To illustrate Borexino’s effect on the low energy $P_{ee}$ measurements, the green (dashed) points are calculated without using the Borexino data. The MSW-LMA prediction is also shown for comparison; the band defines the $1-\sigma$ range of the mixing parameter estimate in [15], which does not include the current result.