Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

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$\ddot{I}ƒ$ 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 $\hat{I}{¸}_{12}$, $\hat{I}{¸}_{23}$, $\hat{I}{¸}_{13}$, a mass difference $\hat{I}”m_{32}^2$ and a CP violating phase $\hat{I}{´}_{\mathrm{CP}}$. In this neutrino oscillation scenario, assuming $|\hat{I}”m_{32}^2|=2.4\times 10^{-3}$ $\mathrm{e}{\mathrm{V}}^2$, ${\sin }^2\hat{I}{¸}_{23}=0.5$, and $\hat{I}”m_{32}^2>0$ ($\hat{I}”m_{32}^2<0$), a best-fit value of ${\sin }^{2}2\hat{I}{¸}_{13}$ = $0.140_{-0.032}^{+0.038}$ ($0.170_{-0.037}^{+0.045}$) is obtained at $\hat{I}{´}_{\mathrm{CP}}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of $\hat{I}{¸}_{13}$ from reactor experiments, some values of $\hat{I}{´}_{\mathrm{CP}}$ are disfavored at the 90% CL.

8 pages, 6 figures