Indication of Electron Neutrino Appearance from an Accelerator-produced Off-axis Muon Neutrino Beam

The T2K experiment observes indications of ${\nu }_{\mu }\to {\nu }_e$ appearance in data accumulated with $1.43\times {10}^{20}$ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with $\mathrm{\mid }\mathrm{\Delta }{m}_{23}^2\mathrm{\mid }=2.4\times {10}^{-3}$ eV${}^2$, ${\sin }^{2}2{\theta }_{23}=1$ and ${\sin }^{2}2{\theta }_{13}=0$, the expected number of such events is 1.5$\pm$0.3(syst.). Under this hypothesis, the probability to observe six or more candidate events is 7$\times {10}^{-3}$, equivalent to 2.5$\sigma$ significance. At 90% C.L., the data are consistent with 0.03(0.04)$<{\sin }^{2}2{\theta }_{13}<$ 0.28(0.34) for ${\delta }_{\mathrm{C}\mathrm{P}}=0$ and a normal (inverted) hierarchy.