Charge Asymmetry in Top Quark Pair Production at √s = 13 TeV with the ATLAS Detector

Vallance, Robert ORCID: 0000-0002-6782-1941 (2020). Charge Asymmetry in Top Quark Pair Production at √s = 13 TeV with the ATLAS Detector. University of Birmingham. Ph.D.

 Preview
Vallance2020PhD.pdf
Text - Accepted Version

Abstract

The charge asymmetry in top quark pair production was investigated using 139 fb−1 of proton-proton collision data at √s = 13 TeV with the ATLAS experiment. This was undertaken in the dilepton channel, where two oppositely-charge leptons (electrons or muons), two jets and missing transverse energy from two neutrinos were required. Asymmetries were determined in the full truth-level phase space for the reconstructed ttbar events and a reduced phase space for the two leptons, both using the Fully Bayesian Unfolding method. The asymmetries were calculated inclusively across all the data and differentially in bins of ttbar (or dilepton) transverse momentum, mass and longitudinal velocity. The inclusive tttbar asymmetry is 0.0081 ± 0.0041, which is 2.0σ above zero and 1.1σ above the Next-to-Leading Order Standard Model expectation of 0.00351 ± 0.00006. The agreement improves to 0.4σ with a higher order prediction. The inclusive leptonic asymmetry is 0.0058 ± 0.0014, which is 4.1σ above zero, showing strong evidence for charge asymmetry, and 2.8σ above the Next-to-Leading Order expectation of 0.00192 ± 0.00006. Again, the agreement improves with a higher order prediction to 1.3σ. The dilepton ttbar results are being combined with those in the lepton + jets channel and limits will be placed on an Effective Field Theory coupling coefficient that characterises effects beyond the Standard Model.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Supervisor(s):
Supervisor(s)EmailORCID
Watson, Miriammiriam.watson@cern.chorcid.org/0000-0002-9724-2684
Hillier, StephenSteve.Hillier@cern.chorcid.org/0000-0002-7599-6469
Bracinik, Jurajjb@hep.ph.bham.ac.ukorcid.org/0000-0001-5762-3477