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

TL;DR

The T2K experiment provides evidence for electron neutrino appearance from a muon neutrino beam, suggesting a non-zero value of the mixing angle θ13 with 2.5 sigma significance based on observed events.

Contribution

This paper reports the first indication of electron neutrino appearance in an accelerator experiment, providing constraints on neutrino oscillation parameters.

Findings

Observed 6 candidate events, exceeding the expected 1.5 events under no appearance.

The probability of observing 6 or more events under the null hypothesis is 0.007.

Results are consistent with a non-zero θ13 mixing angle at 90% confidence level.

Abstract

The T2K experiment observes indications of $\nu_\mu\rightarrow \nu_e$ appearance in data accumulated with $1.43\times10^{20}$ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with $|\Delta m_{23}^2|=2.4\times10^{-3}$ 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$0.3(syst.). Under this hypothesis, the probability to observe six or more candidate events is 7$\times10^{-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 $\delta_{\rm CP}=0$ and a normal (inverted) hierarchy.