Associated production of a top-quark and a Higgs-boson in the <em>H</em>→ττ decay channel in <em>pp</em> collisions at √<em>s</em> = 13 TeV using the ATLAS detector

Abstract

Since the discovery of the Higgs-boson in 2012, experiments at the LHC have focused on studying and precisely measuring this particle's properties. Of particular interest is the Yukawa coupling between the Higgs-boson and the heaviest known elementary particle, the top-quark. Besides the coupling's magnitude, its relative sign with respect to the Higgs-boson's coupling to gauge bosons is also of importance. Both aspects can be probed by analysing the associated production of a top-quark and a Higgs-boson (<em>tH</em>). In the Standard Model, a positive sign of this Yukawa coupling is predicted, causing a destructive interference in the <em>tH</em> channel, thereby lowering the cross-section. The simultaneous presence of large background processes makes the search for <em>tH</em> events particularly challenging. However, certain hypotheses beyond the Standard Model allow for an inverted coupling, resulting in a constructive interference that would significantly enhance the <em>tH</em> cross-section. This thesis probes both hypotheses by investigating the <em>tH</em> production in <em>H</em>→ττ decay channels, using <em>pp</em> collisions at √<em>s</em> = 13 TeV, recorded by the ATLAS detector. The analysis is conducted in channels with either one or two hadronically decaying tau-leptons. After applying a common preselection, a categorical neural network is used in each channel to improve the background rejection and thereby to enhance the sensitivity of the analysis. To ensure reliable results, the modelling of all dominant background processes is validated and corrected in dedicated control regions. The cross-section estimation is performed via a binned profile likelihood fit, testing both the Standard Model and the inverted coupling hypothesis.