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_eff=3.5 MeV, the lowest analysis threshold yet achieved with water Cherenkov detector data. In units of 10⁶ cm^-2 s^-1, the total flux of active-flavor neutrinos from B8 decay in the Sun measured using the neutral current (NC) reaction of neutrinos on deuterons, with no constraint on the B8 neutrino energy spectrum, is found to be Φ_NC=5.140_-0.158^+0.160(stat)_-0.117^+0.132(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 B8 neutrino flux and the energy-dependent ν_e survival probability provides a measure of the total B8 neutrino flux Φ_⁸B=5.046_-0.152^+0.159(stat)_-0.123^+0.107(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 θ₁₂=34.06_-0.84^+1.16 degrees and Δm₂₁²=7.59_-0.21^+0.20×10^-5 eV². The global value of Φ_⁸B is extracted to a precision of _-2.95^+2.38%. In a three-flavor analysis the best fit value of sin²θ₁₃ is 2.00_-1.63^+2.09×10^-2. This implies an upper bound of sin²θ₁₃<0.057 (95% C.L.).