Measurement of the production cross section of a Higgs boson in combination with two top quarks at the ATLAS experiment

In this thesis a novel method for quality control of the cooling half-rings in the pixel endcaps is described. The half rings are used to provide the cooling for pixel modules in the new Inner Tracker of the ATLAS experiment that will operate during the runs of the High Luminosity LHC. In the quality control, they are analysed using infrared measurements during cooling processes. Defective half rings can be found and rejected during the serial production. The sensitivity of this method is shown for small ring pieces with known implanted defects, as well as for a half ring prototype. The measurement of the ttH production cross section with H! bb decays for the full Run II data set, corresponding to an integrated luminosity of 139.0 fb-1 at a centre of mass energy of p s = 13 TeV, is discussed. The signal strength with respect to the Standard Model expectation is measured to be: (ttH = 0.84(+0.49-0.44) (+0.21-0.21 stat.)(+0.44 -0.39 sys.) , which corresponds to an observed (expected) significance of 1.9 sigma (2.2sigma). Additionally, a new analysis strategy to measure the CP-nature of the t-H-coupling in the ttH production is described. This analysis is expected to exclude the purely CP-odd scenario at almost 95% confidence level.