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## New Directions in Top Physics

Schell, Torben Karl

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The top quark plays an important role for many aspects of particle physics. The coupling of the Higgs boson to top quarks is a key parameter to probe electroweak symmetry breaking and is important for the evolution of the Higgs potential to high energies. In addition, many models of physics beyond the Standard Model predict heavy particles that decay to top-quark pairs. Furthermore, the unexplained hierarchy of fermion masses culminates in the large top-quark mass. In this thesis, we consider resonance searches based on top quarks in the fully hadronic final state. We employ multivariate techniques in form of boosted decision trees and add several improvements to the original HEPTopTagger algorithm. These modifications and extensions result in the new HEPTopTagger2. The achieved improvements are used to estimate the precision to which the top Yukawa coupling can be measured at a future 100 TeV proton-proton collider in the fully hadronic final state of $t\bar{t}H production. We find that at such a collider a precision measurement of the top Yukawa coupling to 1% should be possible. The statistical precision is backed up by demonstrating that in the ratio$\sigma(t\bar{t}H)/\sigma(t\bar{t}Z)\$ theoretical uncertainties cancel to below-percent level. Finally, we propose a Froggatt-Nielsen-type model to address the hierarchy of fermion masses in the Standard Model and determine current and projected bounds on the available parameter space.