Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in SNO

Abstract

The Sudbury Neutrino Observatory (SNO) is a 1,000 tonne heavy water Cerenkov‐based neutrino detector situated 2,000 meters underground in INCO’s Creighton Mine near Sudbury, Ontario, Canada. For the neutrinos from 8B decay in the Sun SNO observes the Charged Current neutrino reaction sensitive only to electron neutrinos and others (Neutral Current and Elastic Scattering) sensitive to all active neutrino types and thereby can search for direct evidence of neutrino flavor change. Using these reactions and assuming the standard 8B shape, the ve component of the 8B solar flux is φe = 1.76−0.05+0.05(stat.)−0.09+0.09 (syst.) × 106 cm−2s−1 for a kinetic energy threshold of 5 MeV. The non‐ve component is φμτ = 3.41−0.45+0.45(stat.)−0.45+0.48 (syst.) × 106 cm−2s−1, 5.3σ greater than zero, providing strong evidence for solar ve flavor transformation. The total flux measured with the NC reaction is φNC = 5.09−0.43+0.44(stat.)−0.43+0.46 (syst.) × 106 cm−2s−1, consistent with solar models. For charged current events, assuming an undistorted 8B spectrum, the night minus day rate is 14.0% ± 6.3%−1.4+1.5% of the average rate. If the total flux of active neutrinos is additionally constrained to have no asymmetry, the ve asymmetry is found to be 7.0% ± 4.9%−1.2+1.3%. A global solar neutrino analysis in terms of matter‐enhanced oscillations of two active flavors strongly favors the Large Mixing Angle (LMA) solution.