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

TL;DR

The T2K experiment observes indications of ν (μ) → ν(e) appearance in data accumulated with 1.43×10(20) protons on target, and under this hypothesis, the probability to observe six or more candidate events is 7×10(-3), equivalent to 2.5σ significance.

Abstract

The T2K experiment observes indications of $ν_μ\rightarrow ν_e$ 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 $|Δm_{23}^2|=2.4\times10^{-3}$ eV$^2$, $\sin^2 2θ_{23}=1$ and $\sin^2 2θ_{13}=0$, 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}$, equivalent to 2.5$σ$ significance. At 90% C.L., the data are consistent with 0.03(0.04)$<\sin^2 2θ_{13}<$ 0.28(0.34) for $δ_{\rm CP}=0$ and a normal (inverted) hierarchy.

Figures (6)

• Figure 1: Predicted neutrino fluxes at the far detector, in absence of oscillations. The shaded boxes indicate the total systematic uncertainties for each energy bin.
• Figure 2: Beam centering stability in horizontal (x, South--North) and vertical (y, Down--Up) directions as a function of time, as measured by INGRID. Errors shown are only statistical. The dashed lines correspond to a change of beam direction by $\pm$1 mrad.
• Figure 3: Measured muon momentum of $\nu_\mu$ CC candidates reconstructed in the FGD target. The data are shown using points with error bars (statistical only) and the MC predictions are in histograms shaded according to their type.
• Figure 4: Distribution of invariant mass $M_{inv}$ when each event is forced to be reconstructed into two rings. The data are shown using points with error bars (statistical only) and the MC predictions are in shaded histograms, corresponding to oscillated $\nu_{e}$ CC signal and various background sources for $\sin^22\theta_{13}=0.1$. The last bin shows overflow entries. The vertical line shows the applied cut at 105 MeV/$c^2$.
• Figure 5: Same as Fig. \ref{['fig:polfit']} for the reconstructed neutrino energy spectrum of the events which pass all $\nu_{e}$ appearance signal selection criteria with the exception of the energy cut. The vertical line shows the applied cut at 1250 MeV.