Synesthesia : the art of seeing QCD colour : a measurement of colour flow in top-quark pair events

• Main Author: Wilk, Fabian
• Other Authors: Pilkington, Andrew
• Published: University of Manchester 2018
• Subjects: 500
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.756852

In the Summer of 2015, the Large Hadron Collider resumed operation after a two-year shutdown, providing proton-proton collisions at a record energy of 13 TeV. This thesis presents two measurements of data recorded by the ATLAS detector in proton-proton collisions at the LHC. The first is a measurement of the top-quark pair production cross-section. A dataset of 85 inverse picobarn recorded during an early operating period at the new centre-of-mass energy is used. The cross-section is measured in the single-lepton final state using a high-momentum electron or muon. Measured cross-section values from the two sub-channels and the combination are all consistent within their uncertainties with predictions from theoretical calculations. The second analysis using ATLAS data is a measurement of colour flow in top-quark pair events based on a dataset of 36.1 inverse femtobarn. This measurement also uses the single-lepton final state. Observables are constructed from the jet-pull vector, a momentum-weighted angular moment of a jet, which are sensitive to the colour flow in the signal event topology. Two scenarios are considered: the two jets originating from the hadronically decaying W boson and the two b-tagged jets from the top-quark decays. Observables derived from the former are sensitive to colour flow of a colour singlet while those derived from the latter probe the overall top-quark pair colour-flow. The measured observables are corrected for detector effects by unfolding them to particle level. Normalised unfolded distributions are compared to theoretical predictions taken from simulation and the agreement is quantified. Good agreement between the measured data and the predictions is observed for some combinations of observables and predictions. However, none of the predictions describes data well across all measured observables. A model with exotic colour flow is also constructed. The data favours the predictions from simulation according to the Standard Model over the model with exotic colour flow.