Cross-section measurement of top quark pair production with additional b-jets in the eμ channel at √s = 13 TeV with the ATLAS detector

This thesis presents the cross-section measurement of top quark pair (t$\overline{t}$) production in association with additional b -jets (t$\overline{t}$ + b-jets) in proton-proton collisions at a centre-of-mass energy √s = 13 TeV at the Large Hadron Collider. The analysis is performed in the eμ decay channel of t$\overline{t}$ using a dataset comprising an integrated luminosity of 36.1 fb${}^{-1}$ collected by the ATLAS detector in the years 2015 and 2016. The selected events consist of one electron, one muon and at least two b-jets. To minimize the corrections from model dependence the cross-sections are measured in a fiducial phase space which is chosen close to the detector acceptance of the physics objects. Results are presented at particle level in the form of inclusive and differential cross-sections of t$\overline{t}$ final states with at least one (t$\overline{t}$b) or two (t$\overline{t}$b$\overline{b}$) additional b-jets. Comparisons of the measurements to various state-of-the-art theoretical predictions are made. The measured fiducial inclusive cross-section in t$\overline{t}$b and t$\overline{t}$b$\overline{b}$ final states are σ${}_{t\overline{t}b}$ = 181±5(stat)±24(syst) fb σ${}_{t\overline{t}b\overline{b}}$ = 27±3(stat)±7(syst) fb Estimates of the QCD production of additional b-jets obtained after subtracting the contribution from t$\overline{t}$X(b$\overline{b}$) where X can be either Z or Higgs boson are σ${}_{t\overline{t}b}^{Data-t\overline{t}X}$ = 177±5(stat)±24(syst) fb σ${}_{t\overline{t}b\overline{b}}^{Data-t\overline{t}X}$ =25±3(stat)±7(syst) fb The measured inclusive fiducial cross-sections generally exceed the t$\overline{t}$b$\overline{b}$ predictions from various next-to-leading-order matrix element calculations matched to a parton shower but are compatible within the uncertainties. The experimental uncertainties in the measurement are smaller than the uncertainties in the predictions. The differential measurement of t$\overline{t}$ + b-jets is carried out for the first time by the ATLAS collaboration in this analysis. The normalised differential cross-sections as a function of b-jet multiplicity, kinematic observables of the events and properties of b-jet pairs are shown. The observed differential distributions are compared with a multitude of theoretical predictions and a good agreement with data is found for most of them.