Study of the kaon contribution to the T2K neutrino beam using neutrino interactions in the Near Detector

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Abstract

T2K is a long-baseline neutrino oscillation experiment. It uses an accelerator- produced neutrino beam, whereby a beam of protons impinges on a nuclear target, producing kaon and pion mesons that decay to neutrinos. The main neutrino detectors are situated at 2.50 off-axis from the centre of the beam. An accurate flux prediction for this off-axis beam is crucial to achieve the sensitivity required for the goals of T2K. External experiments reduce the major flux uncertainty (hadronic interactions in the target), but are inherently independent of the real and variable beamline conditions of T2K. Therefore, in situ measurements are required to validate the flux. This thesis uses data from the T2K near detector (ND280) to validate the ux prediction. The normalisation of K+-originating neutrinos at the ND280 is measured. The K+ beam component is important since K+ daughters dominate the high energy part of the 𝜈μ beam and contribute to the intrinsic 𝜈e contamination. As many aspects of the beam simulation affect this measurement, including the hadron production at the target and the off-axis angle, it is used to validate the entire system. The November 2010 to March 2011 data set is used, corresponding to 7:837 x 1019 protons on target. 𝜈μ charged-current interactions are selected (with 86.3% purity) using the ND280 tracker and binned according to the momentum and angle of the muon candidate. The Monte Carlo (MC) is fitted to the data to extract the normalisations of both K+ and π+ originating neutrinos, bK and bπ respectively. The flux, cross-section and detector systematic errors are considered. The best fit point is at bK = 0:86 and bπ = 0:78, consistent with the nominal MC at the 1σ level. Additionally, results of the first time calibration of the ND280 detector, primarily of the ECal sub-detector, are presented.

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