E. D. Hallman

Observations of neutral-current $\ensuremath{\nu}$ interactions on deuterium in the Sudbury Neutrino Observatory are reported. Using the neutral current (NC), elastic scattering, and charged current reactions and assuming the standard ${}^{8}\mathrm{B}$ shape, the ${\ensuremath{\nu}}_{e}$ component of the ${}^{8}\mathrm{B}$ solar flux is ${\ensuremath{\varphi}}_{e}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.76}_{\ensuremath{-}0.05}^{+0.05}(\mathrm{stat}{)}_{\ensuremath{-}0.09}^{+0.09}(\mathrm{syst})\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}$ for a kinetic energy threshold of 5 MeV. The non- ${\ensuremath{\nu}}_{e}$ component is ${\ensuremath{\varphi}}_{\ensuremath{\mu}\ensuremath{\tau}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.41}_{\ensuremath{-}0.45}^{+0.45}(\mathrm{stat}{)}_{\ensuremath{-}0.45}^{+0.48}(\mathrm{syst})\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}$, $5.3\ensuremath{\sigma}$ greater than zero, providing strong evidence for solar ${\ensuremath{\nu}}_{e}$ flavor transformation. The total flux measured with the NC reaction is ${\ensuremath{\varphi}}_{\mathrm{NC}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5.09}_{\ensuremath{-}0.43}^{+0.44}(\mathrm{stat}{)}_{\ensuremath{-}0.43}^{+0.46}(\mathrm{syst})\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}$, consistent with solar models.

Solar neutrinos from ${}^{8}\mathrm{B}$ decay have been detected at the Sudbury Neutrino Observatory via the charged current (CC) reaction on deuterium and the elastic scattering (ES) of electrons. The flux of ${\ensuremath{\nu}}_{e}$'s is measured by the CC reaction rate to be ${\ensuremath{\varphi}}^{\mathrm{CC}}({\ensuremath{\nu}}_{e})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.75\ifmmode\pm\else\textpm\fi{}0.07(\mathrm{stat}{)}_{\ensuremath{-}0.11}^{+0.12}(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{theor})\ifmmode\times\else\texttimes\fi{}{10}^{6}{\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}$. Comparison of ${\ensuremath{\varphi}}^{\mathrm{CC}}({\ensuremath{\nu}}_{e})$ to the Super-Kamiokande Collaboration's precision value of the flux inferred from the ES reaction yields a $3.3\ensuremath{\sigma}$ difference, assuming the systematic uncertainties are normally distributed, providing evidence of an active non- ${\ensuremath{\nu}}_{e}$ component in the solar flux. The total flux of active ${}^{8}\mathrm{B}$ neutrinos is determined to be $5.44\ifmmode\pm\else\textpm\fi{}0.99\ifmmode\times\else\texttimes\fi{}{10}^{6}{\mathrm{cm}}^{\ensuremath{-}2}{\mathrm{s}}^{\ensuremath{-}1}$.

The Sudbury Neutrino Observatory (SNO) has measured day and night solar neutrino energy spectra and rates. For charged current events, assuming an undistorted ${}^{8}\mathrm{B}$ spectrum, the night minus day rate is $14.0%\ifmmode\pm\else\textpm\fi{}6.3{%}_{\ensuremath{-}1.4}^{+1.5}%$ of the average rate. If the total flux of active neutrinos is additionally constrained to have no asymmetry, the ${\ensuremath{\nu}}_{e}$ asymmetry is found to be $7.0%\ifmmode\pm\else\textpm\fi{}4.9{%}_{\ensuremath{-}1.2}^{+1.3}%$. A global solar neutrino analysis in terms of matter-enhanced oscillations of two active flavors strongly favors the large mixing angle solution.

The Sudbury Neutrino Observatory has precisely determined the total active (${\ensuremath{\nu}}_{x}$) $^{8}\mathrm{B}$ solar neutrino flux without assumptions about the energy dependence of the ${\ensuremath{\nu}}_{e}$ 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 $5.21\ifmmode\pm\else\textpm\fi{}0.27\mathrm{(}\mathrm{stat}\mathrm{)}\ifmmode\pm\else\textpm\fi{}0.38\mathrm{(}\mathrm{syst}\mathrm{)}\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, in agreement with previous measurements and standard solar models. A global analysis of these and other solar and reactor neutrino results yields $\ensuremath{\Delta}{m}^{2}={7.1}_{\ensuremath{-}0.6}^{+1.2}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }{\mathrm{eV}}^{2}$ and $\ensuremath{\theta}={32.5}_{\ensuremath{-}2.3}^{+2.4}$ degrees. Maximal mixing is rejected at the equivalent of 5.4 standard deviations.

Results are reported from the complete salt phase of the Sudbury Neutrino Observatory experiment in which NaCl was dissolved in the $^{2}\mathrm{H}$${}_{2}\text{O}$ (``${\text{D}}_{2}\text{O}$'') target. The addition of salt enhanced the signal from neutron capture as compared to the pure ${\text{D}}_{2}\text{O}$ detector. By making a statistical separation of charged-current events from other types based on event-isotropy criteria, the effective electron recoil energy spectrum has been extracted. In units of ${10}^{6}{\text{cm}}^{\ensuremath{-}2}{\text{s}}^{\ensuremath{-}1}$, the total flux of active-flavor neutrinos from $^{8}\mathrm{B}$ decay in the Sun is found to be ${4.94}_{\ensuremath{-}0.21}^{+0.21}{\text{(stat)}}_{\ensuremath{-}0.34}^{+0.38}\text{(syst)}$ and the integral flux of electron neutrinos for an undistorted $^{8}\mathrm{B}$ spectrum is ${1.68}_{\ensuremath{-}0.06}^{+0.06}{\text{(stat)}}_{\ensuremath{-}0.09}^{+0.08}\text{(syst)}$; the signal from (${\ensuremath{\nu}}_{x},e$) elastic scattering is equivalent to an electron-neutrino flux of ${2.35}_{\ensuremath{-}0.22}^{+0.22}{\text{(stat)}}_{\ensuremath{-}0.15}^{+0.15}\text{(syst)}$. These results are consistent with those expected for neutrino oscillations with the so-called large mixing angle parameters and also with an undistorted spectrum. A search for matter-enhancement effects in the Earth through a possible day-night asymmetry in the charged-current integral rate is consistent with no asymmetry. Including results from other experiments, the best-fit values for two-neutrino mixing parameters are $\ensuremath{\Delta}{m}^{2}=({8.0}_{\ensuremath{-}0.4}^{+0.6})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ eV${}^{2}$ and $\ensuremath{\theta}=33.{9}_{\ensuremath{-}2.2}^{+2.4}$ degrees.