arXiv subjects

Manfred Kraus

Publications and source records attributed to Manfred Kraus.

29 records · Page 2Linked to original sources

arXiv2021

$t\bar{t}b\bar{b}$ at the LHC: On the size of corrections and $b$-jet definitions

We report on the calculation of the next-to-leading order QCD corrections to the production of a $t\bar{t}$ pair in association with two heavy-flavour jets. We concentrate on the di-lepton $t\bar{t}$ decay channel at the LHC with $\sqrt{s}=13$ TeV. The computation is based on $pp \to e^+ ν_e\, μ^-\barν_μ\, b\bar{b} \,b\bar{b}$ matrix elements and includes all resonant and non-resonant diagrams, interferences and off-shell effects of the top quark and the $W$ gauge boson. As it is customary for such studies, results are presented in the form of inclusive and differential fiducial cross sections. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. We additionally study the impact of the contributions induced by the bottom-quark parton density. Results presented here are particularly relevant for measurements of $t\bar{t}H(H\to b\bar{b})$ and the determination of the Higgs coupling to the top quark. In addition, they might be used for precise measurements of the top-quark fiducial cross sections and to investigate top-quark decay modelling at the LHC.

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arXiv2021

NLO QCD corrections to off-shell ${t\bar{t}W^\pm}$ production at the LHC: Correlations and Asymmetries

Recent discrepancies between theoretical predictions and experimental data in multi-lepton plus $b$-jets analyses for the $t\bar{t}W^\pm$ process, as reported by the ATLAS collaboration, have indicated that more accurate theoretical predictions and high precision observables are needed to constrain numerous new physics scenarios in this channel. To this end we employ NLO QCD computations with full off-shell top quark effects included to provide theoretical predictions for the ${\cal R}= σ_{t\bar{t}W^+}/σ_{t\bar{t}W^-}$ cross section ratio at the LHC with $\sqrt{s}=13$ TeV. Depending on the transverse momentum cut on the $b$-jet we obtain $2\% -3 \%$ theoretical precision on ${\cal R}$, which should help to shed some light on new physics effects that can reveal themselves only once sufficiently precise Standard Model theoretical predictions are available. Furthermore, triggered by these discrepancies we reexamine the charge asymmetry of the top quark and its decay products in the $t\bar{t}W^\pm$ production process. In the case of charge asymmetries, that are uniquely sensitive to the chiral nature of possible new physics in this channel, theoretical uncertainties below $15\%$ are obtained. Additionally, the impact of the top quark decay modelling is scrutinised by explicit comparison with predictions in the narrow-width approximation.

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arXiv2020

The Matrix Element Method as a tool for precision and accuracy

The Matrix Element Method is a promising multi-variate analysis tool which offers an optimal approach to compare theory and experiment according to the Neyman-Pearson lemma. However, until recently its usage has been limited by the fact that only leading-order predictions could be employed. The imperfect approximation of the underlying probability distribution can introduce a significant bias into the analysis which requires a major calibration for the method when applied to parameter determination. Moreover, estimating theoretical uncertainties by scale variation may yield unreliable results. We present the extension of the Matrix Element Method to next-to-leading order in QCD applicable to LHC data defined by common jet algorithms. The accuracy gain is illustrated by simulating a top-quark mass determination from single top-quark events generated with POWHEG+PYTHIA. Additionally, the method's potential for BSM parameter determination is demonstrated by simulating the extraction of a CP-violating Top-Yukawa coupling from events of single top-quarks in association with a Higgs boson.

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arXiv2020

The simplest of them all: $t\bar{t} W^\pm$ at NLO accuracy in QCD

Recent measurements of the $pp\to t\bar{t}W^\pm$ process in multi-lepton final states, as performed by the ATLAS collaboration in the context of the Higgs boson studies in the $t\bar{t}H$ channel, have shown discrepancies between theoretical predictions and experimental data. Such discrepancies have been observed both in the overall normalisation as well as in the modelling of the $t\bar{t}W^\pm$ process. With the goal of understanding and resolving the modelling issues within the SM $t\bar{t}W^\pm$ process we report on the state-of-the-art NLO QCD computation for this process. Specifically, we calculate higher-order corrections to the $e^+ ν_e \,μ^-\barν_μ\, e^+ ν_e \, b\bar{b}$ and $e^- \barν_e \, μ^+ ν_μ\, e^- \barν_e \, b\bar{b}$ final state at the LHC with $\sqrt{s}=13$ TeV. In the computation off-shell top quarks are described by Breit-Wigner propagators, furthermore, double-, single- as well as non-resonant top-quark contributions along with all interference effects are consistently incorporated at the matrix element level. Results at NLO QCD accuracy are presented in the form of fiducial integrated and differential cross sections for two selected renormalisation and factorisation scale choices and three different PDF sets. The impact of the top quark off-shell effects on the $t\bar{t}W^\pm$ cross section is also examined by an explicit comparison to the narrow-width approximation.

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arXiv2019

Matrix Element Method at NLO for (anti-)$\mathbf{k_t}$-jet algorithms

In this article, we present a method to calculate a posteriori event weights at next-to-leading-order (NLO) QCD accuracy for a given jet event defined by the (anti-)$k_t$ algorithm relying on the conventional $2\to 1$ recombination. This is an important extension compared to existing Monte-Carlo tools which generate jet events together with the corresponding weight but do not allow one to calculate the weight for a given event. The method can be used to generate unweighted events distributed according to the fixed-order NLO cross section. In addition, the method allows one to calculate NLO accurate weights for events recorded by experiments. The potential of this ability is illustrated by applying the Matrix Element Method (MEM) to single top-quark events generated with POWHEG in combination with Pythia. For the first time, a systematic study of parton shower effects within the MEM is provided. The method is completely general and can be applied to arbitrary LHC processes.

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arXiv2019

Exploring BSM Higgs couplings in single top-quark production

In this article we study a Standard Model extension modifying the top-quark Yukawa coupling to the Higgs boson by allowing a mixture of CP-odd and -even couplings. Single top-quark production in association with an additional Higgs boson provides a natural laboratory to search for such extensions. However, because of the small cross section the experimental analysis is challenging. Already the measurement of the cross section for this process is highly non-trivial. Furthermore, using only cross section measurements, a certain parameter region would escape detection. Using an explicit BSM scenario we show that employing the Matrix Element Method a precise measurement becomes feasible. Ignoring signal detection efficiencies an integrated luminosity of about 20 fb^-1 would allow a discovery. Assuming signal detection efficiencies at the level of a few percent a potential signal could be established in the high luminosity phase of the LHC.

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arXiv2019

Cross section ratios as a precision tool for $t\bar{t}γ$

We report on our recent calculation for the off-shell ttA process and its potential for precision measurements by constructing ratios of total and differential cross sections. Precise theoretical predictions for these ratios can help to constrain new physics contributions in the top-quark sector of the Standard Model.

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arXiv2017

On the impact of off-shell contributions on the top quark mass extraction in ttj events

In this contribution we present a systematic comparison of the full off-shell calculation for pp -> ttj in the dileptonic decay channel with a description based on the narrow width approximation. The aim of this study is to estimate the size of off-shell and non-resonant contributions to the shape of differential distributions, that will be utilized in the top quark mass extraction using the template fit method.

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arXiv2016

NLO QCD off-shell effects for top pair production with a jet in the dilepton channel

A short summary of a NLO QCD calculation for the top-quark pair production with a jet in the dilepton decay channel is presented. The calculation takes into account all double, single, non-resonant and interference contributions. Thus, NLO QCD corrections are computed for the process pp -> e+ ve mu- vm~ b b~ j + X. This calculation provides the first complete description, which overcomes the narrow width approximation for ttj process in the dilepton decay channel. Results for the total cross section and differential distributions for the LHC at sqrt(s) = 8 TeV are shown.

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arXiv2016

NLO calculations matched with the Nagy-Soper parton shower

An MC@NLO-like matching of NLO QCD calculations with the Nagy-Soper parton shower is briefly summarised. Uncertainties and ambiguities of the matching scheme are shortly discussed. A few results for the pp -> ttj + X production process at the LHC with sqrt(s) = 8 TeV are also shown. All results have been obtained using the Nagy-Soper parton shower implementation in the DEDUCTOR program together with the HELAC-NLO framework.

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arXiv2015

pp -> ttj + X matched to the Nagy-Soper parton shower at NLO QCD

We briefly summarize the Nagy-Soper parton shower and the MC@NLO-like matching scheme. Results obtained using HELAC-NLO framework in conjunction with DEDUCTOR for top quark pair production in association with one hard jet at the LHC are presented. A comparison of our results with other matching schemes and other parton showers is also discussed for various observables.

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