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, at greater than 5?? significance, the observation of ?????????e oscillation for the first time in the world. Combining the T2K ?????????e oscillation measurement with the latest findings on oscillation parameters including the world average value of ??13 from reactor experiments, the constraint on the value of ??CP at the 90% confidence level is obtained. This constraint on ??CP is an important step towards the discovery of CP violation in the lepton sector.