Probing the top quark couplings within the ATLAS detector and EFT global fits

This thesis consists of three parts. The first part explores deviations from the Standard Model (SM) expectations in high-energy physics observables in the top quark electroweak sector using an effective field theory (EFT) extension. This study is greatly motivated by the latest differential cross-section precision measurements provided by the ATLAS experiment, as well as including data from the CMS detector and other experiments such as Tevatron or LEP/SLC; which allow for a rigorous characterisation of the electroweak interactions of the top quark. The resulting in the limits on chosen Wilson coefficients (related to new physics) range from $\pm0.35$ to $\pm8$ TeV$^{-2}$, in agreement with the SM expectation. The results improve previous studies and highlight the need for new observables to constrain EFT coefficients. The second part presents the search for the leptonic charge asymmetry ($A_c^\ell$) in $t\bar{t}W^\pm$ production using 139 fb$^{-1}$ proton-proton collision data collected by ATLAS during Run 2. Due to the unique properties of $t\bar{t}W^\pm$ production, the charge asymmetry is expected to be large and to showcase a significant sensitivity to a reduced set of four-fermion EFT operators, and to the chiral nature of possible new physics in this process. The reconstructed charge asymmetry is found to be $A_c^\ell = -0.12 \pm 0.14 (\text{stat.}) \pm 0.05 (\text{syst.})$. The result is unfolded to particle level (PL) in a fiducial region which is chosen to be close to the reconstruction level region to minimise acceptance effects. At particle-level, this asymmetry is $A_c^\ell(\text{PL}) = -0.11 \pm 0.17 (\text{stat.}) \pm 0.05 (\text{syst.})$. Both values are consistent with the SM expectation: $A_c^\ell(\text{SM}) = -0.084 \, ^{+0.005}_{-0.003} \,\text{(scale)} \pm 0.006 \,\text{(MC stat.)}$ and $A_c^\ell(\text{PL,SM}) = -0.063 \, ^{+0.007}_{-0.004} \,\text{(scale)} \pm 0.004 \,\text{(MC stat.)}$. A focus on improving the accuracy of Monte Carlo (MC) generators for simulating experimental observables is also studied. In particular, the MC modeling of the ttW sample is tested and validated to reduce theoretical uncertainties in LHC results. In the final part, the thesis investigates the Yukawa coupling of the top quark to the Higgs boson. This coupling is the largest one in the SM and is expected to be the most sensitive to new physics effects. In this regard, it can be used to probe and extract limits on a well-known limitation of the SM: the insufficient CP-violating terms to explain, for example, the matter-antimatter asymmetry of the early universe. MC simulations with a CP-violating term are produced, and exclusion limits on the CP mixing angle ($\alpha$) are extracted using also data collected by ATLAS during Run 2. The CP-odd hypothesis ($\alpha=90$) is excluded at 3.9 standard deviations, and a lower limit of $|\alpha| > 43^\circ$ is established at a 95% confidence level.