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

Abstract

The T2K experiment observes indications of $\hat{I}{½}_{\hat{I}}¼\to \hat{I}{½}_e$ 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 $|\hat{I}”m_{23}^2|=2.4\times 10^{-3}$ eV${}^2$, ${\sin }^{2}2\hat{I}{¸}_{23}=1$ and ${\sin }^{2}2\hat{I}{¸}_{13}=0$, 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}$, equivalent to 2.5$\ddot{I}ƒ$ significance. At 90% C.L., the data are consistent with 0.03(0.04)$<{\sin }^{2}2\hat{I}{¸}_{13}<$ 0.28(0.34) for $\hat{I}{´}_{\mathrm{CP}}=0$ and a normal (inverted) hierarchy.

20 pages, 6 figures, version published in PRL