Measurement of the rate of $\nu_e + d \to p+p + e^-$ interactions produced by $^8$B solar neutrinos at the Sudbury Neutrino Observatory

Solar neutrinos from the decay of $^8$B have been detected at the Sudbury Neutrino Observatory (SNO) via the charged current (CC) reaction on deuterium and by the elastic scattering (ES) of electrons. The CC reaction is sensitive exclusively to $\nu_e$, while the ES reaction also has a small sensitivity to $\nu_{\mu}$ and $\nu_{\tau}$. The flux of $\nu_e$ from $^8$B decay measured by the CC reaction rate is $\phi^{\rm CC}(\nu_e) = 1.75 \pm 0.07~({\rm stat.})^{+0.12}_{-0.11}~({\rm sys.}) \pm 0.05~({\rm theor.}) \times 10^6~{\rm cm}^{-2} {\rm s}^{-1}$. Assuming no flavor transformation, the flux inferred from the ES reaction rate is $\phi^{\rm ES}(\nu_x)=2.39\pm 0.34~({\rm stat.}) ^{+0.16}_{-0.14}~({\rm sys.}) \times 10^6~{\rm cm}^{-2} {\rm s}^{-1}$. Comparison of $\phi^{\rm CC}(\nu_e)$ to the Super-Kamiokande Collaboration precision value of $\phi^{\rm ES}(\nu_x)$ yields a $3.3\sigma$ difference, assuming the systematic uncertainties are normally distributed, providing evidence that there is a non-electron flavor active neutrino component in the solar flux. The total flux of active $^8$B neutrinos is thus determined to be $5.44\pm 0.99 \times 10^6~{\rm cm}^{-2} {\rm s}^{-1}$, in close agreement with the predictions of solar models.