Mirror Mirror on the Wall, which Higgs is the oddest of them all: Exploring the Top-Higgs Interconnection with ATLAS

With the ground-breaking discovery of a new scalar particle, the Higgs boson, in 2012 by ATLAS and CMS, the standard model (SM) of particle physics has been completed. Despite this success, many open questions on the fundamental laws of nature remain unanswered. Among these are how exactly particles acquire their mass and why there is more matter than antimatter in the universe.One of the most promising avenues to approach these questions is to explore the relation between the Higgs boson and the heaviest known elementary particle: the top quark. Due to its large mass, the top is expected to play a special role in the mechanism of electroweak symmetry breaking. In order to shed light onto this mechanism, understanding the coupling between the top and the Higgs in great detail and exploring the charge and parity (CP) nature of the Higgs are essential. While the SM Higgs boson is CP-even, many models beyond the SM require a CP-odd component. Higgs-top couplings are expected to provide an unambiguous probe of CP-mixed states.I will explore for the first time all processes in which a direct determination of the top-Higgs interconnection is feasible, in particular events where the Higgs is produced in association with 1, 2 and 4 top quarks. These are among the most challenging channels at the LHC. I will pioneer a comprehensive programme, consisting of the development of powerful event-reconstruction methods and improved boosting techniques, allowing the first exploitation of novel variables in a beyond-state-of-the-art cross-process analysis, thus unravel the CP-properties in the top-Higgs interaction.The ultimate goal of the project is the precise direct measurement of the top-Higgs Yukawa coupling, and the first determination of the CP-nature of the Higgs boson in fermion interactions. Confronting these results with the SM and models that go beyond the SM will yield an unprecedented insight into the origin of mass of elementary particles.