Departmental Oral Examination (Thesis Title: "Search for the production of Higgs bosons in association with top quarks and decaying into bottom quark pairs with the ATLAS detector")
Physics and Astronomy
The Standard Model of particle physics (SM) describes mass generation of fundamental particles via the Brout-Englert-Higgs mechanism. It predicts Yukawa interactions between the Higgs boson and fermions, with interaction strengths proportional to the fermion masses. The largest Yukawa coupling is that of the top quark, and its value has implications in particle physics and beyond. As the SM is not a complete theory of nature, detailed measurements of its predictions are a mandatory step towards improving the understanding of nature.
This dissertation presents a search for Higgs boson production in association with a top quark pair, a process directly sensitive to the top quark Yukawa coupling. The search uses 36.1 fb−1 of data at √s = 13 TeV, collected with the ATLAS detector at the Large Hadron Collider (LHC) in 2015 and 2016. It is designed for Higgs boson decays to bottom quarks, and decays of the top quark pair resulting in one or two electrons or muons. The discrimination between the signal Higgs boson production process and background processes, dominated by the production of top quark pairs, is performed with multivariate analysis techniques. The matrix element method is used and optimized for this search. Possible machine learning extensions of the method are investigated to help overcome its large computational demand. The obtained ratio of the measured cross-section for the signal Higgs boson production process to the prediction of the SM is µ = 0.84 (+0.64, −0.61). The expected sensitivity of an extension of the search, using 139.0 fb−1 of data collected between 2015–2018, is 3.3σ. Data collected between 2016–2018 is also used in a measurement of the ATLAS muon trigger system efficiency.
A statistical combination of searches for Higgs bosons produced in association with top quark pairs is performed, including the search for Higgs boson decays to bottom quarks and additional final states. The combination results in the observation of this Higgs boson production process with an observed significance of 5.4σ, compared to an expected sensitivity of 5.5σ. It experimentally establishes Yukawa interactions in the SM.