Measurement of the top-quark mass with the ATLAS detector using $t\bar{t}$ events with a boosted top quark

As the heaviest fundamental particle observed to date, the top quark plays a crucial role in particle physics. Precise determination of its mass is essential for testing the internal consistency of the Standard Model and probing potential new physics. This thesis presents a measurement of the top-quark mass using the full Run 2 dataset, corresponding to an integrated luminosity of 140 fb$^{−1}$, collected during proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The analysis focuses on top-antitop-quark pair decays that produce a highly energetic top quark, reconstructed using a single large-radius jet. The average invariant mass of this jet is used to extract the top-quark mass using a profile likelihood fit, incorporating two additional observables to constrain and reduce systematic uncertainties. This approach yields the most precise top-quark mass measurement by ATLAS in a single channel to date: $172.95\pm0.53$ GeV. In addition, this thesis presents the development and integration of the Fast Track Finder algorithm into the ATLAS trigger system. This represents the first successful implementation of a CPU-based charged-particle track reconstruction chain within the ATLAS trigger that is intended for use during the operation of the High-Luminosity Large Hadron Collider, which is scheduled to commence in 2030.