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

Abstract

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