In this thesis the author contributes to the analysis of neutrino beam data collected between 2010 and 2013 to identify ν_μ→ν_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 inT2K neutrino oscillation measurement. In the thesis, the measurement of ν_μ→ν_e  oscillation in the T2K (Tokai to Kamioka) experiment is presented and a new constraint on δ_CP is obtained. This measurement and the analysis establish, at greater than 5σ significance, the observation of ν_μ→ν_e  oscillation for the first time in the world. Combining the T2K ν_μ→ν_e  oscillation measurement with the latest findings on ... Read moreoscillation parameters including the world average value of θ13 from reactor experiments, the constraint on the value of δ_CP at the 90% confidence level is obtained. This constraint on δ_CP is an important step towards the discovery of CP violation in the lepton sector.