Precision Measurements of Higgs Boson Couplings in the Diphoton Decay Channel with Run-2 of the ATLAS Detector

In the second run of the Large Hadron Collider, proton-proton collisions were recorded with the ATLAS detector at a center-of-mass energy of 13 TeV, almost twice that of the previous run. This dramatic increase in energy has enabled physicists to target and precisely measure rare production modes of the recently-discovered Higgs boson for the first time. Due to its special role in explaining the origin of fermion and boson masses, measuring the various interactions of the Higgs is of high priority to the ATLAS collider physics program. The diphoton decay channel of the Higgs (H → γγ) offers one of the best probes of many such interactions due to its relatively clean decay signature and the ATLAS detector’s high-quality photon resolution. Two major physics analyses are discussed in this dissertation, both of which target this decay channel. Both use the full Run 2 dataset gathered by the ATLAS detector, collected during the 2015-2018 data-taking period and corresponding to a time-integrated luminosity of 139 fb−1 . The first of these analyses is a dedicated measurement of the CP properties of the top-Higgs Yukawa coupling, targeting Higgs production in association with a top quark pair (ttH) as well as Higgs production in association with a single top quark (tW H and tHjb). Two Boosted Decision Trees are developed, one to separate ttH + tW H + tHjb signal from QCD continuum diphoton background and another to separate CP-even-like signal events from CP-odd-like signal events. 20 categories are constructed using the outputs of these two decision trees, and a likelihood fit is performed across all categories. An upper limit is placed on the tH production cross-section of 11.6 times the Standard Model expectation, and the observed ttH significance is measured to be 5.2 σ, marking the first observation of the ttH process in a single Higgs decay channel. The fully CP-odd top Yukawa coupling scenario is excluded with a significance of 3.9 σ, while the CP mixing angle is constrained to be |α| ≥ 43◦ at 95% confidence level. In the second analysis, a variety of Higgs production modes are characterized using the Simplified Template Cross-Sections (STXS) framework. In total, the cross-section times the diphoton decay branching ratio is measured in 88 categories corresponding to 27 theoretically-motivated STXS kinematic regions. The inclusive Higgs boson production cross-section in the Higgs boson rapidity range |yH | < 2.5 times the diphoton decay branching ratio is measured to be 127 ± 10 fb. In addition, the ggF + bbH production cross-section is measured to be 104 ± 11 fb, the VBF production cross-section is measured to be 10.7+2.1−1.9 fb, the WH production cross-section is measured to be 6.4+1.5−1.4 fb, the ZH production cross-section is measured to be −1.2−1.0 +1.1 fb, and the ttH + tH production cross-section is measured to be 1.2+0.4−0.3 fb. The compatibility between the measurement and the expected value corresponds to a p-value of 3%, a 1.9σ deviation from the Standard Model. However, when the WH and ZH processes are combined into a single VH process, its cross-section times branching ratio is measured to be 5.9 ± 1.4 fb, the compatibility between the measurement and the expected value corresponds to a p-value of 50%, and no significant deviation from the Standard Model is observed. Similarly, when the STXS bins are considered individually, the p-value of the measurement is 60%, and no significant deviation from the Standard Model is observed. In addition, an upper limit is placed on the tH production cross-section of 8.2 times the Standard Model expectation, the strictest limit placed on this process to date.