Top-antitop quark cross-section measurement in proton-proton collisions at √s = 13 TeV with the atlas experiment at the cern large hadron collider

Abstract

The top quark was first discovered at the Tevatron proton-antiproton collider in 1995 and was first observed in proton-proton collisions at the LHC by both the ATLAS and CMS experiments in 2010. The top quark is the most massive elementary particle in the framework of the Standard Model, which has a large coupling to the Higgs boson and unique role in the electroweak symmetry breaking. Moreover, the top quark is an important background for several analyses involving the Higgs boson and searches for new physics. Therefore, having an accurate understanding and value of inclusive production cross-section of tt is vital. The analysis developed by the candidate and presented in this dissertation has been subject of the first publication of the ATLAS experiment on top quark physics: the measurement of the top-antitop (tt) total production cross-section. The analysis is updated here with the full dataset, corresponding to a data sample of 139 fb-1, of 13 TeV proton-proton collisions collected from LHC Run 2 with ATLAS detector. This measurement uses two kinds of events: first, events with an opposite-charge electron-muon pair in the final states and jets are selected with no missing energy, requiring at least one of the jets to be tagged as coming from the hadronisation of a b-quark. Second, events with an opposite-charge same lepton pair (ee/uu) in the final states and jets are selected with missing energy, requiring at least one of the jets to be tagged as coming from the hadronisation of a b-quark. The cross-section is extracted, using a cut and count method for which an accurate background estimation is crucial, to be st t = 816 1 stat 59 1th. syst 29 exp. syst pb and st t = 799 2 stat 84 th. syst 33 exp. syst pb in eu channel and combined ee/uu channel, respectively. The result of eu channel is in excellent agreement with theoretical predictions and measurements done by ATLAS and CMS experiments, and eµ channel is considered as the cleanest and best channel for tt production cross-section measurement. Besides, a test of the Standard Model is performed by comparing Monte Carlo simulated samples with the experimental results. The Standard Model turned out to be extremely successful in describing the experimental results.