Extraction of top-Yukawa coupling from $t\bar{t}$ cross-section in single-leptonic final state with $\sqrt{s}$ = 13TeV data from the ATLAS detector

In the Standard Model (SM) of particle physics, the interaction of fermions with the Higgs field is referred to as the Yukawa interaction, the strength of which is proportional to the mass of the fermions. The top quark, being the heaviest fermion, exhibits the largest value of the Yukawa coupling strength. Conventionally, the pp→t¯tH process has been extensively used to extract the top-Yukawa coupling. However, this thesis aims to measure the ratio of the top-Yukawa coupling strength over its SM value, i.e., Yt=gt/gSMt using differential distributions of the t¯t cross-section. This method has an advantage that the obtained Yt is independent of the Higgs coupling to other particles. The analysis is conducted on the single-leptonic decay channel using 2015−2018 data with a centre-of-mass energy of √s=13TeV from the ATLAS detector at the Large Hadron Collider (LHC), corresponding to an integrated luminosity of 140fb−1. The presence of a virtual Higgs boson between the two top quarks modifies the t¯t differential cross-section. The electroweak correction, which is a function of Yt, starts to enter the cross-section at one-loop order. The largest Yt sensitivity is observed in the regions close to the t¯t production threshold energy, i.e., at low t¯t invariant mass (mt¯t) regions. An angular dependence is also observed on the scattering angle of the top quark in the t¯t rest frame, at large mt¯t values. Events are reconstructed with at least four jets in the final state, two of them with a requirement that they originate from a B-hadron decay. An algorithm is devised to specifically reconstruct the mt¯t for each event. A detailed statistical analysis is then conducted, taking into account all relevant backgrounds, whose contribution to the total Monte Carlo prediction is very small. A profile likelihood fit is performed on data using binned mt¯t distributions for different Yt values. Due to a linear dependence of the electroweak corrections on Y2t, the fit is performed with Y2t as the parameter of interest. The analysis is dominated by systematic uncertainties, with the jet energy modelling and resolution uncertainties contributing the most to the resultant error on Y2t. This is followed by theoretical modelling uncertainties on the t¯t sample. The fit results in an expected Y2t of 1.0+1.8−1.7, with the observed Y2t of 2.3+1.8−1.7. An upper limit on Yt at 95\% confidence level is extracted using the obtained Y2t. An observed upper limit of 2.32 on Yt is thus achieved, with the expected upper limit of 2.06. The resulting Yt from this method is less precise, but complementary to the Yt obtained from other measurement methods due to its sensitivity to a heavy Higgs sector.