Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

TL;DR

The T2K experiment has observed electron neutrino appearance in a muon neutrinos beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV, corresponding to a significance of 7.3σ.

Abstract

The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of 7.3$σ$ when compared to 4.92 $\pm$ 0.55 expected background events. In the PMNS mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles $θ_{12}$, $θ_{23}$, $θ_{13}$, a mass difference $Δm^2_{32}$ and a CP violating phase $δ_{\mathrm{CP}}$. In this neutrino oscillation scenario, assuming $|Δm^2_{32}| = 2.4 \times 10^{-3}$ $\rm eV^2$, $\sin^2 θ_{23} = 0.5$, and $Δm^2_{32} >0$ ($Δm^2_{32} <0$), a best-fit value of $\sin^2 2 θ_{13}$ = $0.140^{+0.038}_{-0.032}$ ($0.170^{+0.045}_{-0.037}$) is obtained at $δ_{\mathrm{CP}}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of $θ_{13}$ from reactor experiments, some values of $δ_{\mathrm{CP}}$ are disfavored at the 90% CL.

Figures (6)

• Figure 1: The muon momentum distribution for the ND280 CC-0$\pi$ sample (upper). The black points represent the data, the blue histogram shows the MC prediction before data constraint, and the red histogram shows the MC prediction after constraint. The lower plot shows the ratio of data to MC for the pre- and post-constraint cases.
• Figure 2: The $\ln(L_{\pi^0}/L_e)$ vs $m_{\pi^0}$ distribution is shown for both signal $\nu_e$-CC events (boxes) and background events containing a $\pi^0$ (blue scale). The red line indicates the location of the $\pi^0$ rejection cut. Events in the upper right corner are rejected.
• Figure 3: The ($p_e$, $\theta_e$) distribution for $\nu_e$ candidate events with the MC prediction using the primary method best-fit value of $\textrm{sin}^22\theta_{13} = 0.140$ (normal hierarchy).
• Figure 4: The $E_{\nu}^{\rm rec}$ distribution for $\nu_e$ candidate events with the MC prediction at the best fit of $\textrm{sin}^22\theta_{13} = 0.144$ (normal hierarchy) by the alternative binned $E_{\nu}^{\rm rec}$ analysis.
• Figure 5: The 68% and 90% CL allowed regions for sin$^22\theta_{13}$, as a function of $\delta_{\mathrm{CP}}$ assuming normal hierarchy (top) and inverted hierarchy (bottom). The solid line represents the best fit sin$^2 2\theta_{13}$ value for given $\delta_{\mathrm{CP}}$ values. The values of sin$^2\theta_{23}$ and $\Delta m^{2}_{32}$ are varied in the fit with the constraint from Abe:2013fuq. The shaded region shows the average $\theta_{13}$ value from the PDG2012 Beringer:1900zz.