Measurement of the t¯tH(b¯b) process using the ATLAS detector with an effective field theory interpretation

The associated production of a Higgs boson with a top-quark pair (t¯tH) provides a direct probe of the top–Higgs Yukawa coupling, the largest coupling in the standard model. Constraining this process is essential for testing the standard model and for probing possible contributions from physics beyond the standard model. This thesis presents a measurement of t¯tH production in the H → b¯b decay channel, using the full Run 2 dataset, 140 fb−1 of proton-proton collision data at a centreof-mass energy of √ s = 13 TeV, collected by the ATLAS detector at the LHC. Included in this analysis are: novel machine learning methods, improved modelling and reconstruction techniques, dedicated high pT uncertainty extrapolation and more. An excess of events over the non-t¯tH background is found with an observed (expected) significance of 4.6 (5.4) standard deviations. The t¯tH cross-section is measured to be σttH¯ = 411+101 −92 fb = 411 ± 54 (stat.) +85 −75 (syst.) fb, for a Higgs boson mass of 125.09 GeV, consistent with the standard model prediction of 507+35 −50 fb. The cross-section is also measured differentially in bins of the Higgs boson transverse momentum within the Simplified Template Cross-Section framework. This analysis is then re-interpreted in the Standard Model Effective Field Theory framework. The three main operators governing the ttH production in this framework, OtG, Otϕ, OϕG, are investigated and constraints are applied to their respective Wilson coefficients ctG, ctϕ, cϕG. Improvements in precision of the Wilson coefficients were seen in individual quadratic fits compared to early Run-2 (2016) studies, particularly with ctϕ = 2.55+2.22 −2.15, whose contributions are unique to the t¯tH production mode, sees improvements in precision by a factor of 2.