Constraining the top quark Yukawa coupling in in single-top Higgs production

Abstract

The work presented in this thesis consists of the first search for the associated production of a top quark and a Higgs boson (tH) with the ATLAS detector at the LHC. It also establishes an important contribution to the fast simulation of the ATLAS detector. A novel analysis of the tH process has been developed using the di-photon (H → γγ) decay channel. The analysis is designed to simultaneously optimise sensitivity to the Standard Model (SM) tH production, and to the scenario in which the top quark Yukawa coupling to the Higgs boson has a negative sign. The ratio of the measured and predicted cross-sections is reported as: σtH/σSM tH = 2.6 +4.2 −3.2 = 2.6 +3.7 −2.9 (stat.) +1.6 −1.1 (syst.). (1) An upper limit on the tH cross-section is set at 10 times the SM expectation, and the negative values of the top quark Yukawa coupling are excluded at 2.2σ. At the time of submission, these results apply the most stringent constraints on the tH cross-section and the top quark Yukawa coupling. The muon punch-through component of the ATLAS fast simulation tool, AtlFastIII (AF3), has been developed. AF3 improves the speed of the ATLAS detector simulation by a factor of 5, facilitating the production of large sets of simulated data that aid in future ATLAS physics results. Muon punch-through occurs when particles created in electromagnetic showers in the ATLAS calorimeters are not fully contained. This can result in signatures that may be mis-reconstructed as muons. Accurate modelling of this effect is essential for understanding muon fakes and for hadronic jet calibration.