Search for new physics with top quark pairs in the $\ell$+jets channel at $\sqrt{s}$ =13TeV with the ATLAS experiment at the LHC

The Standard Model of particle physics embodies the accumulated knowledge of the structure and the interaction of ordinary matter. However, questions remain unanswered, which motivate the extension of the model in order to include new particles. These gaps are to be closed in direct and indirect searches, with the top quark, the heaviest known elementary particle, playing a central role. In the thesis at hand data from the ATLAS detector at the particle accelerator LHC, which were recorded in 2015 and 2016 at a center of mass energy of 13 TeV, are analyzed. Using actual examples, both approaches to the search for evidence of new physical phenomena in the $\ell$+jets decay channel of top-antitop quark pair production are presented. In the first part of this thesis the mass spectrum of top-antitop pairs is analyzed in a direct search for hints of new physics via resonances of theoretically predicted heavy particles. The main focus of the analysis is the estimation of the W+jets background, which is determined in an external control region and estimated from data based on the jet flavor dependent W$^{+}$/W$^{−}$ asymmetry in the cross-section. Since no significant deviations from the standard model expectations are observed within the top-antitop mass spectrum, exclusion limits in the range of 0.5 to 3.8 TeV can be set for selected theoretical models. In the second part, an indirect search is presented by performing a precision measurement of the charge asymmetry in the top-antitop pair production. In addition to the inclusive measurement, a differential measurement as a function of the invariant top-antitop mass is accomplished, which is considered to be particularly sensitive to boosted top decays in the high mass range. The central question of the measurement, however, is the optimization potential of the analysis regarding the systematic uncertainty by using the previously determined W+jets background. Within the scope of the uncertainties, the results of the charge asymmetry measurements are in line with the expectations according to the Standard Model.