Cross-section measurement of top quark pair production with additional $b$-jets in the $e\mu$ channel at $\sqrt{s}$ = 13 TeV with the ATLAS detector

This thesis presents the cross-section measurement of top quark pair ($t\bar{t}$) production in association with additional $b$-jets ($t\bar{t}+b$-jets) in proton-proton collisions at a centre-of-mass energy, $\sqrt{s}$ = 13 TeV at the Large Hadron Collider. The analysis is performed in the $e\mu$ decay channel of $t\bar{t}$ 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 $t\bar{t}$ final states with at least one ($t\bar{t}b$) or two ($t\bar{t}b\bar{b}$) additional $b$-jet(s). Comparisons of the measurements to various state-of-the-art theoretical predictions are made. The measured fiducial inclusive cross-section in $t\bar{t}b$ and $t\bar{t}b\bar{b}$ final states are $\sigma _{t\bar{t}b}=181\pm5\mathrm{(stat)}\pm 24\mathrm{(syst)fb}$ and $\sigma _{t\bar{t}b\bar{b}}=27\pm3\mathrm{(stat)}\pm 7\mathrm{(syst)fb}$. Estimates of the QCD production of additional $b$-jets obtained after subtracting the contribution from $t\bar{t}b\bar{b}$ where $X$ can be either $Z$ or Higgs boson are $\sigma _{t\bar{t}b}^{Data-t\bar{t}X}=177\pm 5 \mathrm{(stat)}\pm 24\mathrm{(syst)fb}$ and $\sigma_{t\bar{t}b\bar{b}}^{Data-t\bar{t}X}=25\pm3\mathrm{(stat)}\pm 7\mathrm{(syst)fb}$. The measured inclusive fiducial cross-sections generally exceed the $t\bar{t}b\bar{b}$ 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 $t\bar{t}+b$-jets is carried out for the first time by the ATLAS collaboration in this analysis. The normalised differential cross-sections as a function of $b$-jets multiplicity, kinematic observables of the events and properties of $b$-jets pairs are shown. The observed differential distributions are compared with multitude of theoretical predictions and a good agreement with data is found for most of them.