Charge Asymmetry in Boosted Top Quark Pair Production in pp Collisions at the ATLAS Experiment

Abstract

In this thesis, the charge asymmetry in top-quark pair (tt) production in proton-proton collisions at the Large Hadron Collider at the centre-of-mass energy 13 TeV is studied using ATLAS Monte Carlo (MC) simulation and the Run-II data, with a focus on dedicated techniques for identification of high-energy (boosted) hadronically-decaying top quarks (boosted top-quark tagging). The performance of a number of top-quark and W -boson tagging algorithms are investigated. Comparisons of MC simulation with data collected in 2015–2016 with an integrated luminosity of 36.1 fb−1 are performed, showing good agreement, demonstrating that it is in principle possible to calibrate arbitrarily complex tagging algorithms using data. The charge asymmetry in the tt production is measured in the single-lepton channel, using the full Run-II ATLAS dataset with the integrated luminosity of 139 fb−1. For the first time at a hadron collider, the evidence of non-zero charge asymmetry is found with the significance of 4σ. The measurement is found to agree with the Standard Model prediction. Finally, a preliminary estimate of the expected statistical and systematic uncertainties of a charge asymmetry measurement in boosted all-hadronic tt events assuming full Run-II is obtained. The obtained estimate suggests that a combination of the boosted all-hadronic and single-lepton channel would lead to a sizeable reduction of the total uncertainty of measurement for tt mass above 1.5 TeV.