Measurement of the Total Active ${}^8\mathrm{B}$ Solar Neutrino Flux at the Sudbury Neutrino Observatory with Enhanced Neutral Current Sensitivity

The Sudbury Neutrino Observatory has precisely determined the total active (${\nu }_x$) ${}^8\mathrm{B}$ solar neutrino flux without assumptions about the energy dependence of the ${\nu }_e$ survival probability. The measurements were made with dissolved NaCl in heavy water to enhance the sensitivity and signature for neutral-current interactions. The flux is found to be $5.21\pm 0.27\mathrm{(}\mathrm{stat}\mathrm{)}\pm 0.38\mathrm{(}\mathrm{syst}\mathrm{)}\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, in agreement with previous measurements and standard solar models. A global analysis of these and other solar and reactor neutrino results yields $\Delta m^2={7.1}_{-0.6}^{+1.2}\times {10}^{-5}{\mathrm{eV}}^2$ and $\theta ={32.5}_{-2.3}^{+2.4}$ degrees. Maximal mixing is rejected at the equivalent of 5.4 standard deviations.

Figure 1

(a) Neutron capture efficiency versus source radial position for the pure ${\mathrm{D}}_2\mathrm{O}$ phase (capture on D) and salt phase (capture on Cl or D) deduced from a ${}^{252}\mathrm{Cf}$ source, with fits to an analytic function (salt) and to a neutron diffusion model (${\mathrm{D}}_2\mathrm{O}$). (b) Event isotropy from data and Monte Carlo calculations of a ${}^{252}\mathrm{Cf}$ source and an ${}^{16}\mathrm{N}$ $\gamma$-ray source.

Figure 2

Distribution of (a) ${\beta }_{14}$, (b) $\cos {\theta }_{\odot }$, and (c) kinetic energy, for the selected events. The CC and ES spectra are extracted from the data using ${\beta }_{14}$ and $\cos {\theta }_{\odot }$ distributions in each energy bin. Also shown are the Monte Carlo predictions for CC, ES, NC plus internal- and external-source neutron events, all scaled to the fit results. The dashed lines represent the summed components. All distributions are for events with $T_{\mathrm{eff}}\ge 5.5\mathrm{MeV}$ and $R_{\mathrm{fit}}\le 550\mathrm{cm}$. Differential systematics are not shown.

Figure 3

Volume-weighted radius ($\rho$) distribution ($\mathrm{\text{acrylic vessel}}=1$). Shown are the distributions for CC, ES, NC plus internal- and external-source neutrons, scaled from the fit, and separately determined external background distributions.

Figure 4

SNO-only neutrino oscillation contours, including pure ${\mathrm{D}}_2\mathrm{O}$ day and night spectra, salt CC, NC, ES fluxes, with ${}^8\mathrm{B}$ flux free and hep flux fixed. The best-fit point is $\Delta m^2=4.7\times {10}^{-5}$, ${tan}^2\theta =0.43$, $f_B=1.03$, with ${\chi }^2/\mathrm{DOF}=26.2/34$.

Figure 5

Global neutrino oscillation contours. (a) Solar global: ${\mathrm{D}}_2\mathrm{O}$ day and night spectra, salt CC, NC, ES fluxes, Super-Kamiokande, Cl, Ga. The best-fit point is $\Delta m^2=6.5\times {10}^{-5}$, ${tan}^2\theta =0.40$, $f_B=1.04$, with ${\chi }^2/\mathrm{DOF}=70.2/81$. (b) Solar global plus KamLAND. The best-fit point is $\Delta m^2=7.1\times {10}^{-5}$, ${tan}^2\theta =0.41$, $f_B=1.02$. In both (a) and (b) the ${}^8\mathrm{B}$ flux is free and the hep flux is fixed.