Physics at ATLAS From Top to Bottom: Jet Substructure in Boosted $t\bar{t}$ Events at 13 TeV and Development of Pixel Detector Modules for the Inner Tracker Upgrade

Operating at the energy frontier, between 2015 and 2018, CERN's Large Hadron Collider~(LHC) collided protons at an unprecedented centre-of-mass energy of $\sqrt{s}$=13 TeV, allowing for the production of massive particles in great abundance. Over this period, the ATLAS Experiment collected 139 fb$^{-1}$ of data suitable for physics analysis; the largest $pp$ dataset to date. Using this dataset, this thesis presents a measurement of the internal structure of jets arising from the production of the most massive of the Standard Model particles, the top quark, in $t\bar{t}$ pairs, in the case that these jets are produced with high momentum. These results are presented as differential cross-section measurements, unfolded to remove detector effects, and compared to state-of-the-art Monte Carlo simulations. It is found that some observables are modelled poorly by current predictions and that these substructure observables are sensitive to the choice of parton shower modelling and modelling of the final state radiation. Also presented are testbeam studies which investigate the viability of various silicon pixel sensor designs to be used in the upcoming upgrade of the ATLAS tracking system. Here, it is found that all proposed designs meet the required specification for device efficiency after irradiation and are therefore suitable for use in the High-Luminosity LHC.