Observation of Electron Neutrino Appearance from an Off-Axis Muon Neutrino Beam Utilizing the Neutrino Energy Spectrum
T2K (Tokai to Kamiokande) is a long baseline neutrino experiment located in Japan. It uses a 30 GeV proton beam at the J-PARC accelerator in Tokai to produce an intense off-axis muon neutrino beam that travels 295 km through the Earth to Super-Kamiokande. One of its primary goals is to measure neutrino oscillation parameters by directly detecting electron neutrino at Super-Kamiokande that have oscillated from the muon neutrino beam. The measurement of muon neutrino to electron neutrino oscillations are of a particular interest because this mode is sensitive to both mixing angle theta-13 and CP phase delta-cp of the PMNS matrix. Precise measurement of muon neutrino to electron neutrino allows us to explore the possibility of CP violation in the lepton sector.
This talk will describe the recent 2013 electron neutrino appearance oscillation analysis using the reconstructed neutrino energy spectrum by means of a maximum likelihood fit. The data used for this analysis corresponds to 6.57x10^20 POT. A total of 28 electron neutrino candidate events were observed, corresponding to a 7.2 sigma significance of electron neutrino appearance by non-zero theta-13. These result are then combined with the world average value of theta-13 from reactor experiments and some values of delta-cp are disfavored at the 90% CL.