Searches for vector-like quarks with 13 TeV at the ATLAS experiment and development of a boosted-object tagger using a deep neural network

In this thesis, two searches for the pair production of vector-like top (VLT) and vector-like bottom (VLB) quarks are presented using $36.1\,\text{fb}^{-1}$ and $139\,\text{fb}^{-1}$ of data. The data was taken at $\sqrt{s}=13\,\text{TeV}$ in proton-proton collisions with the ATLAS detector, which is one of the major experiments at the Large Hadron Collider at CERN. The decay signature of this search for $T\bar{T}$ and $B\bar{B}$ production is characterized by the presence of a $Z$-boson, which is reconstructed from two opposite-charge same-flavor leptons ($e$ or $\mu$) and is produced alongside a third-generation quark. In addition, the analyses target final-state objects with high transverse momentum, $p_{\text{T}}$. Within the scope of this thesis, the Multi-Class Boosted-Object Tagger (MCBOT) is developed and characterized. It uses a deep neural network, which identifies reclustered large-radius jets originating from $V$-bosons, $H$-bosons, top-quarks and background unambiguously. For the search with $139\,\text{fb}^{-1}$, the multiplicity of hadronic high-$p_{\text{T}}$ objects is exploited by defining multiple regions according to the number of MCBOT- and $b$-tags. No deviation from the Standard Model expectation is found in the observed data for $36.1\,\text{fb}^{-1}$ and $139\,\text{fb}^{-1}$, and exclusion limits on the pair production cross-section and the mass of VLT and VLB quarks are set depending on various branching-ratio assumptions. With $36.1\,\text{fb}^{-1}$, VLT quark masses below $1102\,\text{GeV}$ are excluded with respect to the $(T,B)$ and $(X,T)$ doublet models. For VLB quarks, lower mass limits of $1063\,\text{GeV}$ are set for the $(B,Y)$ doublet. The results of this search significantly contribute to the sensitivity of two combination searches for $T\bar{T}$ and $B\bar{B}$ production performed at the ATLAS experiment with $36.1\,\text{fb}^{-1}$, which constrain the phase-space for vector-like quark existence further. For the full Run-2 search with $139\,\text{fb}^{-1}$ of data, lower limits on the VLT quark mass of $1350\,\text{GeV}$ are observed and VLB quark masses below $1316\,\text{GeV}$ are excluded assuming the mentioned doublet models. The observed limits of this search for $T\bar{T}$ and $B\bar{B}$ production represent the highest exclusion limits to date assuming the $100\%$ branching ratio into $Zt/b$ final states and - in case of VLB quarks - for the $(B,Y)$ doublet model.