Analysis of the $t\bar{t}H$ and $t\bar{t}W$ processes in multi-lepton final states with the ATLAS detector at the LHC

This thesis investigates the properties of the top quark and the Higgs boson using 140 fb$^{-1}$ of proton-proton collision data collected by the ATLAS detector during LHC Run 2 (2015-2018) at $\sqrt{s} = 13$ TeV. The work comprises three main contributions: detector performance studies, the inclusive and differential measurement of the $t\bar{t}H$ cross-section in multi-lepton final states, and the search for the leptonic charge asymmetry in $t\bar{t}W$ production using final states with three charged light leptons.

The first part focuses on the alignment of the ATLAS Inner Detector. Using $Z$ decays into two opposite-charged muons, the track sagitta bias was measured across the detector. Residual distortions were found to be below 0.4 TeV$^{-1}$, and impact parameter biases remained under 0.33 µm (transverse) and 5 µm (longitudinal) throughout Run 2, confirming the alignment's robustness.

The second contribution presents an inclusive and a differential cross-section measurement of the $t\bar{t}H$ production in multi-lepton final states. BDTs and GNNs are used for signal-background separation and Higgs transverse-momentum reconstruction, respectively. The differential cross-section ratios to the SM are measured in three bins of the Higgs transverse momentum: [0, 120), [120, 200) y [200, $\inf$) GeV. Results are found to be in agreement with the Standard-Model prediction. The inclusive signal strength was also observed to be in agreement with the Standard Model, with an observed significance of 2.94$\sigma$.

Finally, the $t\bar{t}W$ process was studied through the leptonic charge asymmetry, measured in events with three charged light leptons. The result of $-0.12 \pm 0.14$, agrees with the Standard-Model prediction and it is severely dominated by data statistics. Future datasets from Run 3 and the HL-LHC are expected to improve the sensitivity of this analysis.