Studies of tt¯H(ττ) event topologies and kinematics with the ATLAS experiment

The tt¯H process grants one of the few opportunities to access the Yukawa coupling between the Higgs field and top quarks on tree level. This thesis examines possibilities for more elabor ate analysis methods going beyond the cut and count approach in the tt¯H(ττ) channel, with a focus on the semileptonic decay of the top quarks and the tau pair. Experience has shown that in the H → ττ channel the shape of the reconstructed invariant mass can be a powerful tool for signal extraction. In the channel considered reconstructing the invariant ττ mass be comes especially challenging as an ambiguity due to two charged leptons in the final state only one of which is a product of the H → ττ decay- arises. Moreover, the missing momenta of the neutrinos cannot be reconstructed easily. To address these problems, the specific topo logies and kinematics of tt¯H(ττ) events are studied. As simple, topological considerations are found to be insufficient for the intended purpose, a likelihood based kinematic fit to fully reconstruct the tt¯ system is employed in order to associate the two leptons correctly with their respective decay systems. Finally, a new likelihood, based on the transverse masses of the leptonically decaying top quark and W boson, is defined and its potential is investigated.