Measurement of Tau Neutrino Appearance and Charged-current Tau Neutrino Cross Section with Atmospheric Neutrinos in Super-Kamiokande

Super-Kamiokande is a 50 kiloton water-Cherenkov detector in Japan, which has been collecting atmospheric neutrino data for more than 20 years. Tau neutrino appearance is expected in atmospheric neutrinos due to neutrino oscillations. The wide span of neutrino energies and neutrino path lengths in atmospheric neutrinos and the large target mass of Super-Kamiokande allow a detection of charged-current tau neutrino interactions in Super-Kamiokande.

This thesis describes a search for atmospheric tau neutrino appearance and a measurement of the charged-current tau neutrino cross section in Super-Kamiokande. A neural network is applied to identify charged-current tau neutrino interactions contained in the Super-Kamiokande detector. Using 5,326 days of atmospheric neutrino data, Super-K measures the tau normalization to be 1.47$\pm$0.32 under the assumption of the normal hierarchy, relative to the expectation of unity for nominal oscillation parameters and assumed charged-current tau neutrino cross section. The result excludes the hypothesis of no-tau-appearance with

a significance level of 4.6$\sigma$, thus giving a direct evidence of neutrino flavor change due to neutrino oscillations. By scaling the cross sections in the simulations to match Super-K data, a flux-averaged charged-current tau neutrino cross section is measured to be $(0.94\pm0.20)\times 10^{-38}$ cm$^{2}$, compared with the theoretical flux-averaged charged-current tau neutrino cross section of $0.64\times 10^{-38}$ cm$^{2}$. This is the second reported measurement of this process. The measured cross section is consistent with the Standard Model prediction.