Abstract
The discovery of the Higgs boson in 2012 by the ATLAS and CMS Collaborations at the Large Hadron Collider in Geneva constituted a cornerstone achievement in particle physics, validating the mechanism of electroweak symmetry breaking in the Standard Model. However, the Standard Model is widely regarded as a low-energy approximation of a more general theory, motivating precision studies of the Higgs sector as a probe for physics beyond the Standard Model. This thesis presents a dedicated analysis of reducible background estimation in the Higgs boson decay channel H →ZZ* →4l. This channel is often referred to as the “golden channel” due to its clean final state and high sensitivity to small deviations from Standard Model predictions. The analysis is performed within the framework of the Standard Model Effective Field Theory (SMEFT), focusing on dimension-six operators that modify the Higgs–gauge boson couplings.
Using the full Run 2 dataset collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 146.9 fb-1, electron backgrounds are studied in detail. Control regions enriched in reducible backgrounds are employed to extract background templates and selection efficiencies, which are then fitted to data to quantify background contributions. These estimates are then extrapolated to the signal region and categorized according to reconstruction channels. The resulting background estimates are incorporated into likelihood scans of SMEFT Wilson coefficients, demonstrating the impact of background modeling on sensitivity to effects beyond the Standard Model.
This work establishes a systematic methodology for reducible background estimation in precision Higgs analyses, ensuring robust interpretation of results in the SMEFT framework. The techniques developed here contribute to the broader ATLAS program of probing Higgs couplings and searching for subtle signatures of new physics.