Constraining the flux and cross section models using carbon and oxygen targets in the off-axis near detector for the 2016 joint oscillation analysis at T2K

Abstract

The T2K experiment is a long baseline neutrino experiment designed to measure various neutrino oscillation parameters, particularly θ₁₃ and θ₂₃, at world-leading precisions. T2K uses muon neutrino and antineutrino beams produced at the Japan Proton Accelerator Research Complex. The off-axis beam strikes near detectors 280 m from the source (ND280) and the water Cherenkov detector, Super-Kamiokande, 295 km away. The far detector performs the primary oscillation analysis, while ND280 provides cross-section and flux constraints to reduce uncertainties in the oscillation analyses. ND280 consists of several different detectors, with the main target mass being the Fine Grained Detectors, one of which provides an active carbon target and one which provides a combined water-carbon target. This thesis describes the fitting methods used for T2K's oscillation analyses and near detector constraints, with a focus on the near detector fit. The addition of water target data in the near detector fit improves cross-section uncertainties at both near and far detectors. The near detector fit is a maximum likelihood fit to observed neutrino interaction rates in ND280 to constrain the cross-section and flux at the far detector. Event cuts identify and separate charged-current neutrino interactions into topology-based samples, with selection uncertainties and correlations handled through extensive detector systematic studies. The near detector fit to data including a water target shows consistent fitted cross-section and flux with previous ND280 fits, and significantly reduces the uncertainty on the neutrino energy spectrum predictions at the far detector. In addition, nuclear model dependencies in the cross-section model used at T2K were investigated using the near detector fit, and the T2K model was shown to be sufficient in accounting for nuclear effects.

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