Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

Abstract

The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of $7.3\sigma$ when compared to $4.92\pm 0.55$ expected background events. In the Pontecorvo-Maki-Nakagawa-Sakata mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles ${\theta }_{12}$, ${\theta }_{23}$, ${\theta }_{13}$, a mass difference $\mathrm{\Delta }{m}_{32}^2$ and a $CP$ violating phase ${\delta }_{CP}$. In this neutrino oscillation scenario, assuming $|\mathrm{\Delta }{m}_{32}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$, ${\sin }^2{\theta }_{23}=0.5$, and $\mathrm{\Delta }{m}_{32}^2>0$ ($\mathrm{\Delta }{m}_{32}^2<0$), a best-fit value of ${\sin }^{2}2{\theta }_{13}=0.140_{-0.032}^{+0.038}$ ($0.170_{-0.037}^{+0.045}$) is obtained at ${\delta }_{CP}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of ${\theta }_{13}$ from reactor experiments, some values of ${\delta }_{CP}$ are disfavored at the 90% C.L.