Solar Neutrino Observations at the Sudbury Neutrino Observatory

Abstract

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.