The Sudbury Neutrino Observatory (SNO) has measured day and night solar neutrino energy spectra and rates. For charged current events, assuming an undistorted [Formula presented] spectrum, the night minus day rate is [Formula presented] of the average rate. If the total flux of active neutrinos is additionally constrained to have no asymmetry, the [Formula presented] asymmetry is found to be [Formula presented]. A global solar neutrino analysis in terms of matter-enhanced oscillations of two active flavors strongly favors the large mixing angle solution.

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Journal Physical Review Letters
Ahmad, Q.R. (Q. R.), Allen, R.C. (R. C.), Andersen, T.C. (T. C.), Anglin, J.D. (J. D.), Barton, J.C. (J. C.), Beier, E.W. (E. W.), … Yeh, M. (M.). (2002). Measurement of Day and Night Neutrino Energy Spectra at SNO and Constraints on Neutrino Mixing Parameters. Physical Review Letters, 89(1). doi:10.1103/PhysRevLett.89.011302