Indication of Electron Neutrino Appearance from an Accelerator-Produced Off-Axis Muon Neutrino Beam

The T2K experiment observes indications of ν_μ→ν_e appearance in data accumulated with 1.43×10²⁰ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with |Δm₂₃²|=2.4×10^-3eV², sin⁡²2θ₂₃=1 and sin⁡²2θ₁₃=0, the expected number of such events is 1.5±0.3(syst). Under this hypothesis, the probability to observe six or more candidate events is 7×10^-3, equivalent to 2.5σ significance. At 90% C.L., the data are consistent with 0.03(0.04)<sin⁡²2θ₁₃<0.28(0.34) for δ_CP=0 and a normal (inverted) hierarchy.