arXiv subjects
Publications and source records attributed to Jonathan Hermann.
In these proceedings we discuss $t\bar{t}H$ production at the Large Hadron Collider in the Higgs characterisation model. We demonstrate the importance of off-shell effects and higher-order QCD corrections for this process and highlight the importance of single-resonant contributions for the production of a $\mathcal{CP}$-odd Higgs boson.
Since its discovery at the Large Hadron Collider in 2012 the Higgs boson has arguably become the most famous of the Standard Model particles and many measurements have been performed in order to assess its properties. Among others, these include measurements of the Higgs boson's ${\cal CP}$ state which is predicted to be ${\cal CP}$-even. Even though a pure ${\cal CP}$-odd state has been ruled out, a possible admixture of a ${\cal CP}$-odd Higgs state has yet to be excluded. In this work we present predictions for the associated production of a leptonically decaying top quark pair and a stable Higgs boson $p p \to e^+ \nu_e\, \mu^- \bar{\nu}_\mu\,b\bar{b}\,H$ with possible mixing between ${\cal CP}$-even and ${\cal CP}$-odd states at NLO in QCD for the LHC with $\sqrt{s} = 13$ TeV. Finite top-quark and gauge boson width effects as well as all double-, single- and non-resonant Feynman diagrams including their interference effects are taken into account. We compare the behaviour of the ${\cal CP}$-even, -odd and -mixed scenarios for the integrated fiducial cross-sections as well as several key differential distributions. In addition, we show that both NLO corrections and off-shell effects play an important role even at the level of integrated fiducial cross-sections and that these are further enhanced in differential distributions. Even though we focus here on the Standard Model Higgs boson, the calculations could be straightforwardly applied to models that have an extended Higgs-boson sector and predict the existence of ${\cal CP}$-odd Higgs-like particles, such as the two-Higgs-doublet model.
In these proceedings we discuss the importance of higher-order corrections and off-shell effects for the calculation of signal strength exclusion limits in $t\bar{t}+$DM searches at the Large Hadron Collider. We present limits for the spin-0 s-channel mediator model using state-of-the-art NLO QCD predictions with full off-shell effects for the relevant $t\bar{t}$ and $t\bar{t}Z$ background processes. These are then compared to less sophisticated predictions using either the narrow-width approximation or LO calculations in order to asses the respective effects.
New Physics searches at the LHC rely very heavily on the precision and accuracy of Standard Model background predictions. Applying the spin-0 $s$-channel mediator model, we assess the importance of properly modelling such backgrounds in $t\bar{t}$ associated Dark Matter production. Specifically, we discuss higher-order corrections and off-shell effects for the two dominant background processes $t\bar{t}$ and $t\bar{t}Z$ in the presence of extremely exclusive cuts. Exclusion limits are calculated for state-of-the-art NLO full off-shell $t\bar{t}$ and $t\bar{t}Z$ predictions and compared to those computed with backgrounds in the NWA and / or at LO. We perform the same comparison for several new-physics sensitive observables and evaluate which of them are affected by the top-quark modelling. Additionally, we make suggestions as to which observables should be used to obtain the most stringent limits assuming integrated luminosities of $300$ fb$^{-1}$ and $3000$ fb$^{-1}$.