OBSERVATION OF A SINGLE TOP QUARK WITH AN ASSOCIATED CHARM QUARK WITH THE ATLAS DETECTOR

Abstract

The analysis presented in this thesis is expected to provide the first observation of single top quark production with an associated charm quark. The data used for the analysis are recorded by the ATLAS detector at the LHC using the proton-proton collisions at a center-of-mass energy of 13 TeV with a full integrated luminosity of 140.1 inverse femtobarn. The recorded data is from the 2015-2018 data-taking period. The analysis is performed by defining a signal region with events consisting of a lepton (electron or muon), zero light jets, one b-tagged jet, one c-tagged jet, and missing transverse energy greater than 25 GeV in the final state and two control regions dominated by top–antitop and W+jets samples. A Boosted Decision Tree (BDT) is trained, and the BDT output is used in the signal region to improve the measurement's sensitivity. A binned likelihood fit is performed simultaneously on the signal region and control regions using the BDT distribution in the signal region and the missing transverse energy distribution in the control regions. The data remain blinded in the signal region, and therefore, only expected results are deduced from the fit. The expected uncertainty on the signal strength is measured to be 6 percent, and the expected significance is more than 17, which is well above the 5 sigma discovery threshold, showing strong sensitivity towards the observation of this process.