Probing the tW b vertex structure in t-channel single-top-quark production and decay in pp collisions at 13 TeV with the ATLAS detector

The top quark, being the heaviest known elementary particle, provides a unique probe for testing the Standard Model (SM) and searching for possible effects of new physics. This thesis presents a detailed study of single top-quark production in the t-channel using proton–proton collision data collected by the ATLAS experiment at the LHC. The work focuses on setting limits on Effective Field Theory (EFT) operators that parameterise potential deviations from the SM in the top-quark electroweak coupling. A novel quadruple-differential analysis is performed, exploiting all four angular variables that describe the full decay chain t → Wb → lνb. The decay rate is expanded in terms of M-function coefficients, which encode the dependence on the angular structure of the process. A simultaneous fit is applied to data and simulated samples using a morphing procedure to interpolate between EFT benchmark points and the SM expectation. This approach allows for a precise extraction of angular coefficients and provides direct sensitivity to anomalous couplings. The analysis achieves the first measurement of the quadruple-differential decay structure in single-top production and places competitive limits on several dimension-six EFT operators affecting the Wtb vertex. The methods developed—particularly the morphing-based fitting framework—offer a general strategy for future precision studies of the top quark and related processes within the EFT formalism.