Low-energy-threshold analysis of the Phase I and Phase II data sets of the Sudbury Neutrino Observatory

Results are reported from a joint analysis of Phase I and Phase II data from the Sudbury Neutrino Observatory. The effective electron kinetic energy threshold used is $T_{\mathrm{eff}}=3.5$ MeV, the lowest analysis threshold yet achieved with water Cherenkov detector data. In units of ${10}^6$ cm${}^{-2}$ ${\mathrm{s}}^{-1}$, the total flux of active-flavor neutrinos from ${}^8\mathrm{B}$ decay in the Sun measured using the neutral current (NC) reaction of neutrinos on deuterons, with no constraint on the ${}^8\mathrm{B}$ neutrino energy spectrum, is found to be ${\Phi }_{\mathrm{NC}}=5.{140}_{-0.158}^{+0.160}{\text{(stat)}}_{-0.117}^{+0.132}\text{(syst)}.$ These uncertainties are more than a factor of 2 smaller than previously published results. Also presented are the spectra of recoil electrons from the charged current reaction of neutrinos on deuterons and the elastic scattering of electrons. A fit to the Sudbury Neutrino Observatory data in which the free parameters directly describe the total ${}^8\mathrm{B}$ neutrino flux and the energy-dependent ${\nu }_e$ survival probability provides a measure of the total ${}^8\mathrm{B}$ neutrino flux ${\Phi }_{{}^8\mathrm{B}}=5.{046}_{-0.152}^{+0.159}{\text{(stat)}}_{-0.123}^{+0.107}\text{(syst)}$. Combining these new results with results of all other solar experiments and the KamLAND reactor experiment yields best-fit values of the mixing parameters of ${\theta }_{12}=34.{06}_{-0.84}^{+1.16}$ degrees and $\Delta m_{21}^2=7.{59}_{-0.21}^{+0.20}\times {10}^{-5}$ eV${}^2$. The global value of ${\Phi }_{{}^8\mathrm{B}}$ is extracted to a precision of ${}_{-2.95}^{+2.38}\%$. In a three-flavor analysis the best fit value of $\sin {}^2{\theta }_{13}$ is $2.{00}_{-1.63}^{+2.09}\times {10}^{-2}$. This implies an upper bound of $\sin {}^2{\theta }_{13}<0.057$ ($95\%$ C.L.).