Aiming For The Top Corner: Measuring top-quark pair production in association with additional jets in highly boosted events using the ATLAS detector

Since beginning operations in 2010 the Large Hadron Collider (LHC) has consistently pushed the boundaries of high energy physics and provided a wealth of proton-proton collision events. Due to the high centre of mass energies achieved, and the QCD nature of proton-proton collisions, the LHC is particularly suited to the production of top-quarks, resulting in the apt nickname of 'top factory'. This thesis outlines the process of making state-of-the-art precision measurements of the top-quark pair production cross-section in events with a high energy top-quark and additional radiation, using data recorded by the ATLAS detector at the LHC from 2015 to 2018. Focus is put on comparing measured values with different QCD predictions showcasing how suited current theories are to estimating complex, multi-scale, and above leading-order processes. Both single and double-differential cross-section measurements as functions of various event kinematic properties are presented with results derived from proton-proton collisions, at a centre-of-mass energy of 13 TeV. Additionally a single differential measurement is used to set limits on new physics using an effective field theory framework.