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%0 Thesis %A Hermann, Jonathan %T Signal and background modeling in BSM studies in the top-quark sector at the LHC %I RWTH Aachen University %V Dissertation %C Aachen %M RWTH-2023-11870 %P 1 Online-Ressource : Illustrationen %D 2023 %Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2024 %Z Dissertation, RWTH Aachen University, 2023 %X This thesis is dedicated to the analysis of modeling effects in associated top-quark pair production processes at the LHC in the context of Beyond the Standard Model (BSM) physics. We provide and discuss state-of-the-art predictions for several BSM signal and Standard Model (SM) background processes and investigate how these are affected by full off-shell effects and higher-order corrections. To facilitate these studies, we present two extensions of the Helac-NLO framework which allow for the inclusion of a <i>CP</i>-mixed top-Higgs Yukawa interaction and contributions from Standard Model Effective Field Theory (SMEFT) operators of dimension six. The phenomenological study is split into three parts, each highlighting a different aspect of BSM analyses. First, we investigate the importance of precisely and accurately modeling the SM background in a search for Dark Matter (DM) production in association with a top-quark pair at the LHC. We compare predictions for the two dominant SM background processes t―t and t―tZ in the dileptonic decay channel of the top-quark pair at leading- (LO) and next-to-leading order (NLO) in QCD, including full off-shell effects and in the narrow-width approximation (NWA). By calculating signal-strength exclusion limits for the t―t+\textDM process for different ways of modeling the SM background, we demonstrate the importance of higher-order corrections and full off-shell effects in BSM searches in exclusive phase-space regions. In addition, we study how these exclusion limits are affected by changes in the integrated luminosity or the renormalization and factorization scale and make suggestions as to which observables should be used to obtain the most stringent possible exclusion limits. In the second analysis, we instead focus on the modeling of a signal process, specifically p p → b ―b e<sup>+</sup> ν<sub>e</sub> μ<sup>−</sup> ―ν<sub>μ</sub> H production with a <i>CP</i>-mixed top-Higgs Yukawa interaction. We investigate the dependence of the integrated fiducial cross section on the mixing angle between the <i>CP</i>-even and <i>CP</i>-odd contributions. Additionally, we compare differential distributions for the production of a <i>CP</i>-even, -mixed, and -odd Higgs boson. We demonstrate that these distributions, as well as the integrated cross sections, are significantly affected by higher-order corrections and off-shell effects and that these effects can have a sizable impact on an observable's sensitivity to the Higgs boson's <i>CP</i> state. In the final phenomenological analysis, we forego the usage of an explicit new physics model altogether and instead study modeling effects in SMEFT. We present results in the NWA and the full off-shell treatment for p p → b ―b e<sup>+</sup> ν<sub>e</sub> μ<sup>−</sup> ―ν<sub>μ</sub> H production at LO, including contributions from the Wilson operators <i>O</i><sub>t ϕ</sub>, <i>O</i><sub>ϕG</sub>, <i>O</i><sub>t G</sub>, and <i>O</i><sub>t W</sub>. We assess the size of off-shell effects in the various linear, squared, and interference contributions that these operators induce at the level of integrated fiducial cross sections and differential distributions. Finally, we investigate the effects of including <i>O</i>( C<sub>tW</sub> / Λ<sup>2</sup> ) contributions to the top-quark decay width and highlight the importance of a consistent expansion in the Wilson coefficients. %F PUB:(DE-HGF)11 %9 Dissertation / PhD Thesis %R 10.18154/RWTH-2023-11870 %U https://publications.rwth-aachen.de/record/975160