Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

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$\sigma$ 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 ${\theta }_{12}$, ${\theta }_{23}$, ${\theta }_{13}$, a mass difference $\Delta m^2_{32}$andaCPviolatingphase$\delta_{$\mathrm{C}\mathrm{P}$}$. In this neutrino oscillation scenario, assuming $\mathrm{\mid }\mathrm{\Delta }{m}_{32}^2\mathrm{\mid }=2.4\times {10}^{-3}$ $\rm eV^2$,$\sin^2 \theta_{23} = 0.5$,and$\Delta m^2_{32} >0$($\Delta m^2_{32} <0$),abest-fitvalueof$\sin^2 2 \theta_{13}$=$0.140^{+0.038}{-0.032}$($0.170^{+0.045}{-0.037}$)isobtainedat$\delta_{$\mathrm{C}\mathrm{P}$}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of ${\theta }_{13}$ from reactor experiments, some values of ${\delta }_{\mathrm{C}\mathrm{P}}$ are disfavored at the 90% CL.