Search for vector-like partners of the top and bottom quarks with the ATLAS experiment

In this dissertation, a search for pair-produced vector-like partners $T$ and $B$ of the top and bottom quarks in events with an isolated lepton, at least four jets, and high missing transverse momentum is presented. Data from proton--proton collisions at a centre-of-mass energy of $\sqrt{s} =$ 13 TeV at the Large Hadron Collider, recorded by the ATLAS experiment from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb$^{-1}$, are analysed. The vector-like quarks are assumed to decay into a $W$, $Z$, or Higgs boson and a third-generation quark. Signal and background events are separated by neural networks, trained for various signal hypotheses. As no significant deviations from the Standard Model expectation are observed, upper limits on the production cross-section of $T$ and $B$ quarks as a function of their masses are derived for various decay scenarios. The strongest limits on the masses are 1.59 TeV for a weak-isospin $(T,B)$ doublet assuming mass degenerate vector-like quarks and 1.47 TeV (1.46 TeV) for exclusive $T\rightarrow Zt$ ($B\rightarrow Wt$) decays. Lower limits on the mass of the $T$ and $B$ quarks are derived for all combinations of branching ratios. Furthermore, improvements of the ATLAS Monte Carlo validation system are discussed, and the results of a dedicated study of the interference effect between top-quark pair production and single top-quark production in association with a $W$ boson are presented in this dissertation.