In this thesis the author contributes to the analysis of neutrino beam data collected between 2010 and 2013 to identify _e events at the Super-Kamiokande detector. In particular, the author improves the pion nucleus interaction uncertainty, which is one of the dominant systematic error sources in T2K neutrino oscillation measurement. In the thesis, the measurement of _e oscillation in the T2K (Tokai to Kamioka) experiment is presented and a new constraint on _CP is obtained. This measurement and the analysis establish,...