Observation of ?_???_e Oscillation in the T2K Experiment

In this thesis the author contributes to the analysis of neutrino beam data collected between 2010 and 2013 to identify ?_µ??_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 ?_µ??_e  oscillation in the T2K (Tokai to Kamioka) experiment is presented and a new constraint on d_CP is obtained. This measurement and the analysis establish, at greater than 5s significance, the observation of ?_µ??_e  oscillation for the first time in the world. Combining the T2K ?_µ??_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 d_CP at the 90% confidence level is obtained. This constraint on d_CP is an important step towards the discovery of CP violation in the lepton sector.