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 $14.0\%\pm 6.3{\%}_{-1.4\mathrm{}}^{+1.5\mathrm{}}\%$ of the average rate. If the total flux of active neutrinos is additionally constrained to have no asymmetry, the ${\nu }_e$ asymmetry is found to be $7.0\%\pm 4.9{\%}_{-1.2\mathrm{}}^{+1.3\mathrm{}}\%$. A global solar neutrino analysis in terms of matter-enhanced oscillations of two active flavors strongly favors the large mixing angle solution.

• Received 19 April 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.011302