Bench, Francis
(2021)
Study of Neutrino and Antineutrino Oscillations in the Three-Flavour PMNS Paradigm at the T2K Experiment: Determination of the CP-Violating Phase and the Search for Muon Antineutrino to Electron Antineutrino Oscillations.
PhD thesis, University of Liverpool.
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Abstract
T2K (Tokai to Kamioka) is an accelerator-based long-baseline neutrino oscillation experiment that aims to study the parameters which govern these oscillations. An almost pure muon neutrino or muon antineutrino beam with flux sharply peaked at a neutrino energy of 0.6 GeV is produced at the J-PARC facility in Japan before passing through a suite of near detectors then travelling 295 km through the Earth’s crust to the 50 ktonne water Cherenkov far detector, Super-Kamiokande. The near detectors, located 280 m from the beam production target, measure the unoscillated flavour composition of the beam and constrain the flux and neutrino-nucleus interaction models. Together, the near and far detectors measure the change in flavour composition, therefore allowing the oscillation parameters to be determined. This thesis presents a simultaneous analysis of observations from five Super-Kamiokande event samples, selected to measure the disappearance of muon neutrinos/antineutrinos and the appearance of electron neutrinos/antineutrinos. Working in the three active neutrino flavour paradigm and using the PMNS parameterisation, constraints are obtained on the oscillation parameters δCP, sin²(θ₂₃), Δm²₃₂, and sin²(θ₁₃) in both mass orderings. Additionally, the muon antineutrino to electron antineutrino oscillation channel was specifically analysed in order to determine whether such events could be fully distinguished from the expected backgrounds and it was found that the background-only hypothesis was disfavoured at a significance of 2.4σ. For all fits to the oscillation parameters, the best-fit was found to be in the normal mass ordering and the corresponding best-fit points and ±1σ ranges are as follows, where the global best-fit measurement on sin²(2θ₁₃) from reactor antineutrino experiments was used as a constraint except when producing the T2K constraint on sin²(θ₁₃). δCP = -2.01 (+0.93 / -0.76) sin²(θ₂₃) = 0.552 (+0.024 / -0.048) Δm²₃₂ = (2.49 ± 0.05) × 10¯³ eV²c⁻⁴ sin²(θ₁₃) = 2.54 (+0.49 / -0.38) × 10¯² For δCP, the best-fit point is close to the maximally CP-violating value of -π/2, and CP-conservation is excluded at close to the 90% confidence level. The observed constraint on δCP is stronger than expected from sensitivity studies, but was found to be compatible with the three-flavour oscillation model. The T2K constraints on Δm²₃₂ and sin²(θ₁₃) are competitive with recent measurements from other experiments, and are world-leading for sin²(θ₂₃) and δCP.
| Item Type: | Thesis (PhD) |
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| Uncontrolled Keywords: | charge partity symmetry violation, CP-Violation, neutrino, neutrino mixing, neutrino oscillation, PMNS, Super-K, Super-Kamiokande, T2K |
| Divisions: | Faculty of Science and Engineering > School of Physical Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 02 Sep 2022 10:27 |
| Last Modified: | 18 Jan 2023 20:46 |
| DOI: | 10.17638/03162728 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3162728 |
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