Measurement of neutrino oscillation parameters at T2K using new neutral current pion samples and probing future sensitivities with Hyper-Kamiokande

This thesis explores approaches to improve neutrino oscillation measurements with the T2K experiment and the next-generation Hyper-Kamiokande (Hyper-K). A unique strength of long-baseline accelerator neutrino experiments is to measure δCP. Observing CP violation at 5σ significance for a large fraction of δCP values requires significant reductions in systematic uncertainties, particularly in the neutrino-nucleus interaction model, as experiments transition from being statistically to systematically limited. Additionally, these experiments aim to precisely measure the absolute mass difference |∆m^2_32| and the mixing angle θ23. Hyper-K’s oscillation sensitivity has been evaluated, comparing scaled T2K systematic models with new constraints from the Intermediate Water Cherenkov Detector (IWCD). The IWCD, designed to be vertically movable, measures the neutrino flux at different angles off the beam axis, enabling precise measurements of the relative ν_e and ν_µ interaction cross sections. The impact of new IWCD cross-section models on oscillation sensitivity is presented. The core of this thesis focuses on implementing new neutral current pion samples into an oscillation analysis. The 1-ring neutral current (NC)1π0 events are backgrounds to the oscillated ν_e signal due to their indistinguishable topologies measured at Super-Kamiokande. A 1-ring π0 sample is reintroduced to recover misidentified ν_e signals, while a high-purity (∼ 71%) and high-efficiency (∼ 74%) 2-ring NC1π0 selection improves modelling of NC1π0 events. Their kinematic properties are studied, and their impact on oscillation sensitivity is evaluated using a Markov Chain Monte Carlo (MCMC) fitting approach. A fit with the new samples to T2K neutrino beam data, using 2.14×10^21 (1.63×10^21) proton-on-target in the (anti-)neutrino beam mode, yields a 1σ credible interval for δCP = [−2.513, −0.880], excluding CP-conservation at a 90% credible level. These results are consistent with the latest T2K analysis, giving a 74% probability for neutrinos under normal mass ordering and a 68% probability that θ23 is in the upper octant.