Measurement of the ${}^8\mathrm{B}$ solar neutrino flux in $\mathrm{SNO}+$ with very low backgrounds

A measurement of the ${}^8\mathrm{B}$ solar neutrino flux has been made using a 69.2 kt-day dataset acquired with the $\mathrm{SNO}+$ detector during its water commissioning phase. At energies above 6 MeV the dataset is an extremely pure sample of solar neutrino elastic scattering events, owing primarily to the detector’s deep location, allowing an accurate measurement with relatively little exposure. In that energy region the best fit background rate is $0.25_{-0.07}^{+0.09}\mathrm{events}/\mathrm{kt}\text{−}\mathrm{day}$, significantly lower than the measured solar neutrino event rate in that energy range, which is $1.03_{-0.12}^{+0.13}\mathrm{events}/\mathrm{kt}\text{−}\mathrm{day}$. Also using data below this threshold, down to 5 MeV, fits of the solar neutrino event direction yielded an observed flux of ${2.53}_{-0.28}^{+0.31}{(\mathrm{stat})}_{-0.10}^{+0.13}(\mathrm{syst})\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, assuming no neutrino oscillations. This rate is consistent with matter enhanced neutrino oscillations and measurements from other experiments.

Figure 1

Energy (a) and direction (b) reconstruction comparison between Monte Carlo simulation and data for ${}^{16}\mathrm{N}$ events. Here $T_e$ is the electron kinetic energy, and $\theta$ is the angle between the reconstructed direction of an event and the direction vector pointing from the source position to the event’s reconstructed position.

Figure 2

Distribution of event direction with respect to solar direction. The systematic error bar includes angular correlated and uncorrelated errors.

Figure 3

Top: The extracted solar neutrino elastic scattering event rate as a function of reconstructed electron kinetic energy $T_e$. Bottom: The same, as a fraction of the expected rate. The red and blue lines show the MC simulation predicted spectrum normalized to the best fit flux and the SNO flux measurement [28], respectively. The uncertainty on the SNO result includes reported uncertainty combined with mixing parameter uncertainties. The black points are the results of the fits to the $\cos {\theta }_{\mathrm{sun}}$ distribution in each energy bin, with error bars indicating the combined statistical and systematic uncertainty, including energy-correlated uncertainty. A horizontal dash is placed on each error bar, indicating the statistics only uncertainty; for all points the statistical error is dominant and the systematic error bar is not visible above the dash.

Figure 4

Distribution of event directions with respect to solar direction for events with energy in the range 6.0–15.0 MeV.