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Malgorzata Worek

Publications and source records attributed to Malgorzata Worek.

At least 37 records · Page 2Linked to original sources
arXiv2022

Production and decay of the Higgs boson in association with top quarks

We report on the calculation of the next-lo-leading order QCD corrections to Higgs boson production and decay in association with top quarks. We consider leptonic decays of top quarks leading to the hadronic process $pp\to e^+ν_e\, μ^-\barν_μ\, b\bar{b}\,H\, (H\to X)$ at the LHC with $\sqrt{s}=13$ TeV. All resonant as well as non-resonant Feynman diagrams, interferences and off-shell effects are included for the top quark and $W$ gauge boson. Decays of the Higgs boson, on the other hand, are included in the narrow-width-approximation. Specifically, we consider Higgs boson decays into $b\bar{b}$, $τ^+τ^-$, $γγ$ and $e^+e^-e^+e^-$. Numerical results are given at the integrated and differential fiducial level for various factorisation and renormalisation scale choices and different PDF sets. We study the main theoretical uncertainties that are associated with neglected higher order terms in the perturbative expansion and with different parametrisations of the PDFs. Furthermore, we examine the size of the off-shell effects by an explicit comparison to the calculation in the full narrow-width-approximation. Finally, the impact of the contributions induced by the bottom-quark parton density is investigated.

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arXiv2021

The impact of top-quark modelling on the exclusion limits in $\boldsymbol{t\bar{t}}+\text{DM}$ searches at the LHC

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$^{-1}$.

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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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arXiv2021

Modelling of top quark decays in $t\bar{t}γ$ production at the LHC

In this proceedings we briefly report on the state-of-the-art NLO QCD computation for the $pp\to t\bar{t}γ$ process in the di-lepton channel. We describe higher-order corrections to the $e^+ν_e \, μ^- \barν_μ\, b\bar{b}γ$ final state at the LHC with $\sqrt{s}=13$ TeV. Off-shell top quarks, double-, single- as well as non-resonant top-quark contributions along with all interference effects are consistently incorporated at the matrix element level. Results are presented in the form of fiducial integrated and differential cross sections. The impact of top quark modelling is scrutinised by an explicit comparison with the results in the narrow-width approximation. Both types of predictions are now available in the HELAC-NLO framework.

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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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arXiv2016

NLO QCD corrections to off-shell ttj production at the LHC

A short summary of results for top-quark pair production in association with a jet with NLO QCD off-shell effects at the LHC is given. The calculation is based on the matrix element for the $pp \to e^+ ν_e μ^-\barν_μb\bar{b} j$ process. All quantum effects are properly accounted for, which amounts to including all Feynman diagrams that result in this final state with interference effects as well as non-zero top-quark width effects. Results for total cross sections and a few differential distributions at the LHC Run1 energy of $\sqrt{s}=8$ TeV are presented. In addition, the size of top-quark off-shell effects is quantified for the total cross section in presence of rather inclusive cuts on all final states. Numerical results shown in this proceedings have been obtained with the HELAC-NLO Monte Carlo program.

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arXiv2016

Top Pair Production with a jet with NLO QCD off-shell effects

A brief summary of the recent next-to-leading order QCD calculation for e+ ve mu- v_mu bb~ j at the LHC is given. Our computation includes all non-resonant contributions, off-shell effects and interferences for top-quarks and W gauge bosons. Some results for integrated and differential cross sections are shown for the LHC Run1 energy of 8 TeV. A significant reduction of the scale dependence is observed, which indicates that the perturbative expansion is well under control. The results are obtained in the framework of the HELAC-NLO system.

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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

Automated NLO/NLL Monte Carlo programs for the LHC

The interpretation of experimental measurements at the LHC requires accurate theoretical predictions for exclusive observables, and in particular the summation of soft and collinear radiation to all orders in perturbation theory. We report on recent progress towards the automated calculation of multi-parton LHC cross sections at next-to-leading order in QCD, including the summation of next-to-leading logarithmic corrections through the combination with parton showers.

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arXiv2013

NLO mass effects in b anti-b b anti-b production at the LHC

The calculation of NLO QCD corrections to the production cross section of two bottom pairs at the LHC is briefly summarized. We describe the pp -> b anti-b b anti-b+X process both in the 4-flavor and 5-flavor scheme and investigate the effect of the finite bottom quark mass. The results for the total cross section and a few differential distributions are presented. They have been obtained with the Nagy-Soper subtraction formalism for real radiation at NLO. A comparison with results based on the traditional Catani-Seymour subtraction is also included.

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arXiv2012

On the next-to-leading order QCD K-factor for top anti-top bottom anti-bottom production at the TeVatron

Motivated by ongoing experimental analyses, we report on the calculation of next-to-leading order QCD corrections to the production of top anti-top pairs in association with two hard b-jets at the Fermilab TeVatron. Besides the total cross section and its scale dependence, a few differential distributions applicable for Higgs boson searches at the TeVaron are given. The QCD corrections with respect to leading order are negative and small. For our main setup they amount to 2%, and remain reasonably stable against changes of cuts. This proves that an integrated next-to-leading order K-factor does not necessarily need to be applied in the background estimation for the top anti-top H -> top anti-top bottom anti-bottom signal process. The distributions show similarly small corrections. The shape of kinematic distributions is distorted at most by about 20% in some corners of the phase space. Even though it is not the main purpose of this paper, we also evaluated the forward-backward asymmetry of the top quark at next-to-leading order.

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arXiv2011

Phenomenological studies of top-pair production at Next-to-Leading order

The calculation of NLO QCD corrections to the $t\bar{t}\to W^{+}W^{-}b\bar{b}\to e^{+}ν_e μ^{-}\barν_μb\bar{b}$ process with complete off-shell effects, is briefly summarized. Besides the total cross section and its scale dependence, a few differential distributions at the TeVatron run II and LHC are given. All results presented in this contribution have been obtained with the help of the HELAC-NLO Monte Carlo framework.

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arXiv2011

Complete off-shell effects in top quark pair hadroproduction with leptonic decay at next-to-leading order

Results for next-to-leading order QCD corrections to the pp(p\bar{p}) -> t \bar{t} -> W^+W^- b\bar{b} -> e^{+} ν_{e} μ^{-} \barν_μ b \bar{b} +X processes with complete off-shell effects are presented for the first time. Double-, single- and non-resonant top contributions of the order {\cal{O}}(α_{s}^3 α^4) are consistently taken into account, which requires the introduction of a complex-mass scheme for unstable top quarks. Moreover, the intermediate W bosons are treated off-shell. Comparison to the narrow width approximation for top quarks, where non-factorizable corrections are not accounted for is performed. Besides the total cross section and its scale dependence, several differential distributions at the TeVatron run II and the LHC are given. In case of the TeVatron the forward-backward asymmetry of the top is recalculated afresh. With inclusive selection cuts, the forward-backward asymmetry amounts to A^{t}_{FB} = 0.051 +/- 0.0013. Furthermore, the corrections with respect to leading order are positive and of the order 2.3% for the TeVatron and 47% for the LHC. A study of the scale dependence of our NLO predictions indicates that the residual theoretical uncertainty due to higher order corrections is 8% for the TeVatron and 9% for the LHC.

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arXiv2010

Recent developments in NLO QCD calculations: the particular case of pp -> ttjj

A very brief status of next-to-leading order QCD calculations is given. As an example the next-to-leading order QCD calculations to the pp -> ttjj processes at the CERN Large Hardon Collider are presented. Results for integrated and differential cross sections are shown. They have been obtained in the framework of the HELAC-NLO system.

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arXiv2010

Next-to-leading order t anti-t plus jets physics with HELAC-NLO

A report on the recent next-to-leading order QCD calculations to t anti-t b anti-b and t anti-t + 2j processes at the CERN Large Hardon Collider is presented. Results for integrated and differential cross sections are given. A significant reduction of the scale dependence is observed in both cases, which indicates that the perturbative expansion is well under control. The results are obtained in the framework of the HELAC-NLO system.

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