The measurement of the Higgs couplings at the LHC with the ATLAS experiment

The objective of this thesis is to study the Higgs boson’s couplings and constrain the shape of the Higgs potential by analyzing events in which two Higgs bosons decay into a pair of bottom quarks and tau leptons. The analysis primarily uses the full proton-proton collision dataset from LHC Run 2 at 13 TeV, with additional exploratory studies using a partial dataset from LHC Run 3 at 13.6 TeV. To enhance sensitivity, advanced event categorization strategies are applied, based on Higgs production mechanisms and tau decay modes. No significant excess over the Standard Model prediction is observed, and stringent limits are set on the Higgs self-couplings and the overall di-Higgs signal strength. A complete reparametrization of the di-tau mass reconstruction is performed using the Missing Mass Calculator, which models the kinematics of neutrinos from tau decays. This approach improves the mass resolution of the di-tau system and may also benefit Higgs measurements involving two taus in the final state, particularly by enhancing separation from dominant background processes. In addition to the di-Higgs analysis, the thesis includes a study of the rare Higgs decay into two muons using the partial Run 3 dataset. This clean final state allows a direct probe of the Higgs coupling to second-generation charged fermions. Several categorization schemes, including multivariate analysis techniques, are explored to improve separation from the dominant Z → μμ background, yielding expected signal sensitivities. Finally, the thesis presents the development and documentation of a data quality monitoring tool for the small-strip Thin Gap Chambers (sTGC) in the ATLAS New Small Wheel muon detector. This tool is essential for ensuring the reliable performance and reconstruction efficiency of the sTGC system, which plays a critical role in analyses involving final-state muons.