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@PHDTHESIS{Hermann:975160,
author = {Hermann, Jonathan},
othercontributors = {Worek, Malgorzata and Krämer, Michael},
title = {{S}ignal and background modeling in {BSM} studies in the
top-quark sector at the {LHC}},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2023-11870},
pages = {1 Online-Ressource : Illustrationen},
year = {2023},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2024; Dissertation, RWTH Aachen University, 2023},
abstract = {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
$\mathcal{CP}$-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
$t\bar{t}$ and $t\bar{t}Z$ 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
$t\bar{t}+\text{DM}$ 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 $p p \to b \bar{b} e^+ \nu_e \mu^-
\bar{\nu}_\mu \, H$ production with a $\mathcal{CP}$-mixed
top-Higgs Yukawa interaction. We investigate the dependence
of the integrated fiducial cross section on the mixing angle
between the $\mathcal{CP}$-even and $\mathcal{CP}$-odd
contributions. Additionally, we compare differential
distributions for the production of a $\mathcal{CP}$-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
$\mathcal{CP}$ 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 $p p \to b \bar{b} e^+ \nu_e \mu^-
\bar{\nu}_\mu \, H$ production at LO, including
contributions from the Wilson operators $\mathcal{O}_{t
\phi}$, $\mathcal{O}_{\phi G}$, $\mathcal{O}_{t G}$, and
$\mathcal{O}_{t W}$. 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 $\mathcal{O}\left( C_{tW} / \Lambda^2 \right)$
contributions to the top-quark decay width and highlight the
importance of a consistent expansion in the Wilson
coefficients.},
cin = {136110 / 130000},
ddc = {530},
cid = {$I:(DE-82)136110_20140620$ / $I:(DE-82)130000_20140620$},
pnm = {DFG project 396021762 - TRR 257: Phänomenologische
Elementarteilchenphysik nach der Higgs-Entdeckung
(396021762) / GRK 2497 - GRK 2497: Physik der schwersten
Teilchen am Large Hadron Collider (400140256)},
pid = {G:(GEPRIS)396021762 / G:(GEPRIS)400140256},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2023-11870},
url = {https://publications.rwth-aachen.de/record/975160},
}
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