Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

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$σ$ when compared to 4.92 $\pm$ 0.55 expected background events. In the PMNS mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles $θ_{12}$, $θ_{23}$, $θ_{13}$, a mass difference $Δm^2_{32}$ and a CP violating phase $δ_{\mathrm{CP}}$. In this neutrino oscillation scenario, assuming $|Δm^2_{32}| = 2.4 \times 10^{-3}$ $\rm eV^2$, $\sin^2 θ_{23} = 0.5$, and $Δm^2_{32} >0$ ($Δm^2_{32} <0$), a best-fit value of $\sin^2 2 θ_{13}$ = $0.140^{+0.038}_{-0.032}$ ($0.170^{+0.045}_{-0.037}$) is obtained at $δ_{\mathrm{CP}}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of $θ_{13}$ from reactor experiments, some values of $δ_{\mathrm{CP}}$ are disfavored at the 90% CL.