Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in the Sudbury Neutrino Observatory

TL;DR

Observations of neutral-current nu interactions on deuterium in the Sudbury Neutrino Observatory are reported, providing strong evidence for solar nu(e) flavor transformation.

Abstract

Observations of neutral current neutrino interactions on deuterium in the Sudbury Neutrino Observatory are reported. Using the neutral current, elastic scattering, and charged current reactions and assuming the standard 8B shape, the electron-neutrino component of the 8B solar flux is 1.76 +/-0.05(stat.)+/-0.09(syst.) x10^6/(cm^2 s), for a kinetic energy threshold of 5 MeV. The non-electron neutrino component is 3.41+/-0.45(stat.)+0.48,-0.45(syst.) x10^6/(cm^2 s), 5.3 standard deviations greater than zero, providing strong evidence for solar electron neutrino flavor transformation. The total flux measured with the NC reaction is 5.09 +0.44,-0.43(stat.)+0.46,-0.43(syst.)x10^6/(cm^2 s), consistent with solar models.

Figures (3)

• Figure 1: Thorium (a) and uranium (b) backgrounds (equivalent equilibrium concentrations) in the D$_2$O deduced by in situ and ex situ techniques. The MnO$_{\rm{x}}$ and HTiO radiochemical assay results, the Rn assay results, and the in situ Cherenkov signal determination of the backgrounds are presented for the period of this analysis on the left-hand side of frames (a) and (b). The right-hand side shows time-integrated averages including an additional sampling systematic uncertainty for the ex situ measurement.
• Figure 2: (a) Distribution of $\cos\theta_{\odot}$ for $R \le 550$ cm. (b) Distribution of the volume weighted radial variable $(R/R_{\rm AV})^{3}$. (c) Kinetic energy for $R \le 550$ cm. Also shown are the Monte Carlo predictions for CC, ES and NC + bkgd neutron events scaled to the fit results, and the calculated spectrum of Cherenkov background (Bkgd) events. The dashed lines represent the summed components, and the bands show $\pm 1\sigma$ uncertainties. All distributions are for events with $T_{\rm eff}$$\geq$5 MeV.
• Figure 3: Flux of ${}^{8}$B solar neutrinos which are $\mu$ or $\tau$ flavor vs flux of electron neutrinos deduced from the three neutrino reactions in SNO. The diagonal bands show the total ${}^{8}$B flux as predicted by the SSM bp2000 (dashed lines) and that measured with the NC reaction in SNO (solid band). The intercepts of these bands with the axes represent the $\pm 1\sigma$ errors. The bands intersect at the fit values for $\phi_{e}$ and $\phi_{\mu\tau}$, indicating that the combined flux results are consistent with neutrino flavor transformation assuming no distortion in the ${}^{8}$B neutrino energy spectrum.