Top-antitop energy asymmetry in jet-associated top-quark pair production at ATLAS

The Standard Model (SM) of particle physics, representing our current understanding of fundamental particles and their interactions, gives correct predictions with an astonishing precision for tens of thousands of measurements. Remaining open questions, like the observed matter-antimatter asymmetry in our universe and observations hinting at the existence of dark matter point to new physics beyond the Standard Model. Observables in the top-quark sector are particularly well suited to probe the SM and many extensions thereof at the electroweak symmetry-breaking scale and beyond. At proton-proton colliders, top-antitop-quark pair production is symmetric at leading order in quantum chromodynamics under the exchange of the top and antitop quarks, while interferences at higher orders in quark-antiquark annihilation create an asymmetry. This charge asymmetry can provide sensitive probes for many models beyond the SM like massive colour-octet states, extra dimensions, flavour-violating gauge bosons or axigluons. In jet-associated top-quark pair production, this asymmetry arises already at leading order in quark-gluon interactions. Furthermore, the final states in jet-associated top-antitop events allow for the definition of a new observable, the energy asymmetry, reflecting the different probability of top and antitop quarks to have the higher energy. The ATLAS experiment at the Large Hadron Collider at CERN collected 139 fb^-1 of proton-proton collision data at a centre-of-mass energy of 13 TeV during Run 2 in the data-taking period 2015-2018, providing the possibility to study less frequent processes like jet-associated top-quark pair production as well as essentially all interactions of the top quark with high statistical precision. This thesis presents the first measurement of the energy asymmetry, performed in the semi-leptonic decay channel in jet-associated top-quark pair events in the so called boosted topology, requiring the hadronically decaying top quark to have a transverse momentum above 350 GeV, and corrected for detector effects with the Fully Bayesian Unfolding (FBU) method. The measured asymmetry is found to be in agreement with the SM prediction at next-to-leading order accuracy in all three bins of the jet-scattering angle. In the central region, where the energy asymmetry is expected to be the highest, the energy asymmetry is measured to be -0.043+-0.020, in agreement with the SM prediction of -0.037+- 0.003. The Standard Model Effective Field Theory (SMEFT) represents a model-independent framework for new physics interpretations. Within the SMEFT framework, the energy asymmetry is especially sensitive to the chiral and colour structure of four-quark operators. The sensitivity of the energy asymmetry is presented in the bounds on Wilson coefficients obtained from one and two-dimensional fits of the predicted to the measured asymmetry. The sensitivity of the energy asymmetry is found to be comparable to that of other observables in the top-quark sector as well as to resolve blind directions in current LHC fits and will thus provide a valuable new input for global SMEFT fits.