Five sample joint neutrino/antineutrino oscillation analysis in T2K

This thesis describes a simultaneous fit to all five of the far detector oscillation samples analysed to date (FHC/RHC 1Rmu, FHC/RHC 1Re and FHC nue CC1pi+) using T2K data taken from 2009-2017 with \sci{1.47}{21}~POT in neutrino mode and \sci{0.76}{21}~POT in antineutrino mode. The aim of this analysis is to search for CP violation by setting limits on \dcp and measure the oscillation parameters \sonethree, \stwothree and \Dm232. The T2K (Tokai to Kamioka) experiment is a long-baseline accelerator neutrino oscillation experiment that is designed to make precision measurements of neutrino oscillation by observing numu disappearance and nue appearance. A numu beam is produced using a 30 GeV proton beam at the J-PARC and is detected by the near detector complex, ND280, and by the far detector, Super-Kamiokande, which is 295 km away from J-PARC. The far detector and one of the near detectors are located in an off-axis direction to the neutrino beam centre, resulting a narrow neutrino energy peak of 0.6 GeV and where the numu survival probability is expected to be minimal (or maximal nue appearance probability). The neutrino fluxes and their uncertainties at the near and far detector are predicted by the Monte Carlo simulation tuned with data from external hadron production experiments. The uncertainties of the neutrino flux parameters and certain neutrino interactions parameters at the far detector are reduced by the measurement at the off-axis near detector. Since the last analysis there has been improvement to event reconstructions at SK which results in 20% increase in nue increased purity in the 1R muon samples. Using data taken from 2009-2017, we observed 240 mu-like, 74 e-like, 15 nue CC1pi+ -like events in neutrino mode, and 68 mu-like, 7 e-like events in antineutrino mode. The best-fit oscillation parameter values for the T2K only fit are $\mathrm{sin}^2\theta_{13} = 0.0296^{+0.0066}_{-0.0052}$, $\mathrm{sin}^2\theta_{23} = 0.511^{+0.036}_{-0.032}$, $|\Delta m^2| = 2.458^{+0.058}_{-0.059}\times10^{-3}\mathrm{eV}^2$ and $\delta_{CP} = -1.72^{+1.04}_{-0.92}$. The best-fit oscillation parameter values including a reactor constraint on \sonethree are $\mathrm{sin}^2\theta_{13} = 0.0224\pm0.0012$, $\mathrm{sin}^2\theta_{23} = 0.530^{+0.028}_{-0.034}$, $|\Delta m^2| = 2.462^{+0.059}_{-0.057}\times10^{-3}\mathrm{eV}^2$ and $\delta_{CP} = -1.64\pm0.62$. These results are quoted from credible intervals presented in the result section. Confidence intervals for delta CP are produced using the Feldman-Cousins method, and confidence intervals of the other oscillation parameters are produced using the constant delta chi square method. The best-fit oscillation parameter values are found to be consistent with previous official results while giving tighter constraints. There is a first indication of CP conservation in neutrino sector excluded at 2 sigma level. The results are also presented showing the correlations that exist between the oscillation parameters.Open Acces