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

Abstract

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 interactions of the recently-discovered Higgs boson for the first time. 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 + tWH + 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 ± 11fb, 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 ± 1.4fb, 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. 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.