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 7.3σ when compared to 4.92 ± 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 θ₁₂, θ₂₃, θ₁₃, a mass difference Δm² ₃₂and a CP violating phase δ_CP. In this neutrino oscillation scenario, assuming |Δm² ₃₂| = 2.4 × 10^-3eV², sin²θ₂₃= 0.5, and Δm² ₃₂> 0 (Δm² ₃₂< 0), a best-fit value of sin²2θ₁₃= 0.140^+0.038 _-0.032(0.170^+0.045 _-0.037) is obtained at δ_CP= 0. When combining the result with the current best knowledge of oscillation parameters including the world average value of θ₁₃from reactor experiments, some values of δ_CPare disfavored at the 90% C.L.