A low energy measurement of the ⁸B solar neutrino spectrum at the Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory has previously shown that ⁸B solar neutrinos undergo flavor transformation between the Sun and the Earth. This work presents a joint analysis of one-third of the pure D₂O and salt phase data sets with an electron energy threshold of 3.5 MeV. A measurement of the neutral current interaction rate, sensitive to all neutrino flavors, gives a total ⁸B flux of 4.79 ± 0.22 (stat) ± 0.13(syst) × 10⁶ cm⁻²s⁻¹. This is in agreement with the prediction of the Standard Solar Model. For the first time we obtain the v[sunscript e] survival probability separately for downward-going (day) and upward-going (night) solar neutrinos. No significant distortion is observed day or night for ⁸B neutrinos with energies greater than 6 MeV. The lack of distortion, but overall suppression of electron neutrinos relative to the total flux, is consistent with matter-induced neutrino transformation in the Sun and the large mixing angle solution.