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T2K is a long-baseline neutrino oscillation experiment. Muon neutrinos are generated by a proton beam, and are detected by near and far neutrino detectors. For a precise measurement of the neutrino oscillation, an accurate understanding of the neutrino beam and the neutrino interaction is essential. Therefore, the measurement of the neutrino beam properties and the neutrino interaction cross sections plays an important role in T2K. Neutrino event rate, neutrino beam direction and neutrino beam width were measured with the near neutrino detector called INGRID. They were confirmed to be stable within the physics requirements and agree well with the T2K neutrino flux predictions. In addition, this measurement reduced the neutrino flux uncertainty from the neutrino beam direction which was originally 7.0% at 1 GeV to 2.5%. The neutrino interaction cross sections in a few GeV region were also measured with INGRID. The charged current inclusive cross sections on iron and hydrocarbon and their cross section ratio were measured to be σFe CC = (1.444± 0.002(stat.) +0.189 −0.157(syst.))× 10−38cm2/nucleon, σCH CC = (1.379 ± 0.009(stat.) −0.147(syst.)) × 10−38cm2/nucleon, and σFe CC/σ CC = 1.047 ± 0.007(stat.) ± 0.035(syst.), at a mean neutrino energy of 1.51 GeV. They agree well with the predicted values by the neutrino interaction models used in T2K. Since the neutrino cross section in a few GeV region is sensitive to the nuclear effect of the target material, this result demonstrates that the nuclear effect is well understood and correctly treated in the models on the 3% level. The measured charged current quasi-elastic cross sections are also consistent with the neutrino interaction models. However, the measured charged current coherent pion production cross section is significantly smaller than the prediction by the original Rein-Sehgal model that is widely used in neutrino experiments. Therefore, we reveal that the original Rein-Sehgal model breaks down in a few GeV region. Since this inconsistency is within the originally assigned uncertainty, all the cross section results from INGRID demonstrate the validity of the neutrino interaction models and their uncertainties in T2K. Finally, a neutrino oscillation analysis was performed using the neutrino flux prediction, the neutrino interaction models and the beam data in the near and far detectors. We performed a joint three-flavor neutrino oscillation analysis of the electron neutrino appearance and the muon neutrino disappearance for the first time. As the result, sin 2θ13 for normal (inverted) neutrino mass hierarchy was determined to be sin 2θ13 = 0.165 +0.044 −0.073 (0.191 +0.056 −0.084). The electron neutrino appearance in a muon neutrino beam was observed because the appearance signal corresponds to a significance of 7.3σ. We also obtained sin θ23 = 0.524 ± 0.058 (0.524 −0.057) and ∆m32 = (2.50±0.11)×10−3eV2 (∆m13 = (2.48±0.11)×10−3eV2) for the normal (inverted) hierarchy. Furthermore, when the T2K result was combined with the result of θ13 measurements by reactor experiments, the 90% confidence level allowed region for δCP was obtained to be −3.59 < δCP < 0.43 (−2.84 < δCP < −0.32) for the normal (inverted) hierarchy.

Measurement of Neutrino Interactions and Three Flavor Neutrino Oscillations in the T2K Experiment