The Sudbury Neutrino Observatory (SNO) is a 1000-tonne heavy water Cherenkov detector. Its usage of \dto as target allows the simultaneous measurements of the $\nu_e$ flux from $^8$B decay in the Sun and the total flux of all active neutrino species through the charged-current and the neutral-current interactions on the deuterons. Assuming the standard $^{ 8}$B shape, the $\nu_e$ component of the $^{ 8}$B solar neutrino flux is measured to be $\phi_e = 1.76^{ +0.05}_{ -0.05}{ (stat.)}^{ +0.09}_{ -0.09} { (syst.)} x 10^6 { \rm cm}^{ -2} { \rm s}^{ -1}$ for a kinetic energy threshold of 5 MeV. The non-\nue component is found to be $\phinumutau = 3.41^{ +0.45}_{ -0.45}{ (stat.)}^{ +0.48}_{ -0.45} { (syst.)} x 10^6 { \rm cm}^{ -2} { \rm s}^{ -1}$. This $5.3\sigma$ difference provides strong evidence for $\nu_{ e}$ flavor transformation in the solar neutrino sector. The total active neutrino flux is measured with the neutral-current reaction at a neutrino energy threshold of 2.2 MeV. This flux is determined to be $\phi_{ NC} = 5.09^{ +0.44}_{ -0.43}{ (stat.)}^{ +0.46}_{ -0.43} { (syst.)} x 10^6 { \rm cm}^{ -2} { \rm s}^{ -1}$, and is consistent with solar model predictions. Assuming an undistorted $^8$B spectrum, the night minus day rate is 14.0$\pm$6.3(stat.)$^{ +1.5}_{ -1.4}$(sys.)\% of the average rate in the charged-current channel. If the total active neutrino flux is constrained to have no asymmetry, the night-day asymmetry in the $\nu_e$ flux is found to be 7.0$\pm$4.9(stat.)$^{ +1.3}_{ 1.2}$(sys.)\%. A global analysis of all the available solar neutrino data in terms of matter-enhanced oscillations of two active flavors strongly favors the Large Mixing Angle (LMA) solution.
Categories: hep-ex nucl-ex