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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 Δm232and a CP violating phase δCP. In this neutrino oscillation scenario, assuming |Δm232| = 2.4 × 10-3eV2, sin2θ23= 0.5, and Δm232> 0 (Δm232< 0), a best-fit value of sin22θ13= 0.140+0.038-0.032(0.170+0.045-0.037) is obtained at δCP= 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 δCPare disfavored at the 90% C.L.

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Physical Review Letters