Measurement of neutrino oscillations using neutrino and antineutrino beams in the NOvA experiment

NOvA is a long-baseline accelerator neutrino oscillation experiment using the NuMI neutrino beam from Fermilab. Its main physics goals are to probe the 3-flavour oscillation parameters: neutrino mass hierarchy, CP-violating phase δ_cp and octant of θ₂₃ mixing angle by observing electron neutrino appearance and muon neutrino disappearance. Two functionally identical detectors are placed off-axis from the centre of the NuMI beam. The near detector at Fermilab is 100 m underground, and the far detector is located on the surface at Ash River, 810 km away from the beam source. The initial neutrino beam spectra are measured using the near detector data and the oscillation parameters are extracted by fitting the observed data to the predicted neutrino spectrum in the far detector. This thesis is centered around how to improve the sensitivity of |Δm²₃₂| and θ₂₃ measurements in the muon neutrino disappearance analysis. NOvA will take data for about 12 years. The operation of the NOvA experiment for each year costs tens of millions of dollars, thus it is valuable to maximise the sensitivity of the analysis. Three samples of muon neutrino events are studied in this thesis to improve the analysis sensitivities. First, higher energy muon neutrinos are investigated by extending the energy range in NOvA's current standard analysis. Second, for the sample of events used in NOvA's existing analysis, a new energy estimator which has been developed to improve the neutrino energy resolution is considered. Furthermore, in addition to binning the events as function of energy and hadronic energy fraction, three particle identifiers are introduced to separate neutrino events by signal purity to reduce the effects from backgrounds. Third, an additional lower purity sample of muon neutrino charged current (CC) events that look similar to neutral current events and have not been included in NOvA's existing analyses have been studied. This thesis reanalyses NOvA's data used in the 2020 analysis, corresponding to an exposure of 13.60×10²⁰ protons on target (POT) in the neutrino beam mode recorded from February 6, 2014 to March 20, 2020, and 12.50×10²⁰ protons on target in the antineutrino beam mode recorded between June 29, 2016 to February 26, 2019. This thesis has implemented a fit to Asimov fake data, generated where sin²θ₂₃ = 0.59 and Δm²₃₂ = 2.5 × 10⁻³ eV². These sensitivity studies show that the uncertainty range of |Δm²₃₂| at 1 σ in the new analysis is reduced by 5.5% and the significance of maximal disappearance rejection improves by 7.7%, compared to the standard analysis. This is equivalent to adding 11-16% more data. This thesis also implements the fit from combining electron neutrino appearance and muon neutrino disappearance. The combined analysis shows that the best fit values are very close to the standard analysis. However, the uncertainty range of Δm²₃₂ at 1 σ is reduced by 3.7% using the new analysis. The maximal disappearance significance is not improved in the new analysis, but the new analysis slightly improves the rejection…

QC0793 Elementary particle physics
neutrino: secondary beam
antineutrino: secondary beam
neutrino: oscillation
NOvA
sensitivity
neutrino/e: particle identification
neutrino: mass: hierarchy
neutrino: mixing angle
neutrino: mass difference

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