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 $|\Delta m_{23}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$, ${sin}^{2}2{\theta }_{23}=1$ and ${sin}^{2}2{\theta }_{13}=0$, the expected number of such events is $1.5\pm 0.3(\mathrm{syst})$. 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 ${\delta }_{CP}=0$ and a normal (inverted) hierarchy.