Jorgen D'Hondt | Searcharxiv

arXiv2026

Constraining new physics in charm quark associated Higgs boson production events using the Standard Model effective field theory approach

As the search for observable deviations from the Standard Model of particle physics remains to be of significant interest, effective field theory (EFT) continues to be a popular method to parametrize such effects. In this work, a first-time investigation is performed of the unique capability of measurements of charm quark associated Higgs boson production (cH) in proton-proton collisions at the CERN Large Hadron Collider to constrain a set of dimension-six EFT operators. The phenomenology of these operators is discussed and a proposed analysis strategy is presented, with a focus on $\mathrm{H}\rightarrow \mathrm{Z}\mathrm{Z}^{*}\rightarrow 4μ$ decays, using a generic detector simulation that is parametrized to reflect the response of the CMS detector at the LHC. From this, expected 95% CL upper limits are derived for the Wilson coefficients of individual operators by considering yield and shape effects in the spectra of the four-muon invariant mass $m_{4μ}$ and leading jet transverse momentum $p_{T}$. Scenarios with simultaneous contributions from two operators are also considered. Finally, potential analysis improvements that may be implemented in an experimental context are outlined.

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arXiv2023

Measurements of the Cross-Section for the $t\bar{t}$ + Heavy-Flavor Production at the LHC

At the LHC, the process of a Higgs boson decaying into bottom or charm quarks produced in association with a pair of top quarks, ttbarH , allows for an empirical exploration of the heavy-flavor quark Yukawa couplings to the Higgs boson. Accordingly, the cross-sections for the $t\bar{t}$ + heavy-flavor production without the appearance of the Higgs boson have been measured at the LHC in various phase spaces using data samples collected in pp collisions at $\sqrt{s}$ = 7, 8 and 13 TeV with the ATLAS and CMS experiments. Flavor ratios of cross-sections of $t\bar{t}$ + heavy-flavors to $t\bar{t}$ + additional jets processes are also measured. In this paper, the measured cross-sections and ratios are reviewed and the prospects with more data are presented.

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arXiv2018

Learning to pinpoint effective operators at the LHC: a study of the $t\bar{t}b\bar{b}$ signature

In the context of the Standard Model effective field theory (SMEFT), we study the LHC sensitivity to four fermion operators involving heavy quarks by employing cross section measurements in the $t\bar{t}b\bar{b}$ final state. Starting from the measurement of total rates, we progressively exploit kinematical information and machine learning techniques to optimize the projected sensitivity at the end of Run III. Indeed, in final states with high multiplicity containing inter-correlated kinematical information, multi-variate methods provide a robust way of isolating the regions of phase space where the SMEFT contribution is enhanced. We also show that training for multiple output classes allows for the discrimination between operators mediating the production of tops in different helicity states. Our projected sensitivities not only constrain a host of new directions in the SMEFT parameter space but also improve on existing limits demonstrating that, on one hand, $t\bar{t}b\bar{b}$ production is an indispensable component in a future global fit for top quark interactions in the SMEFT, and on the other, multi-class machine learning algorithms can be a valuable tool for interpreting LHC data in this framework.

hep-ph↗

arXiv2018

Learning to pinpoint effective operators at the LHC: a study of the $t\bar{t}b\bar{b}$ signature

In the context of the Standard Model effective field theory (SMEFT), we study the LHC sensitivity to four fermion operators involving heavy quarks by employing cross section measurements in the $t\bar{t}b\bar{b}$ final state. Starting from the measurement of total rates, we progressively exploit kinematical information and machine learning techniques to optimize the projected sensitivity at the end of Run III. Indeed, in final states with high multiplicity containing inter-correlated kinematical information, multi-variate methods provide a robust way of isolating the regions of phase space where the SMEFT contribution is enhanced. We also show that training for multiple output classes allows for the discrimination between operators mediating the production of tops in different helicity states. Our projected sensitivities not only constrain a host of new directions in the SMEFT parameter space but also improve on existing limits demonstrating that, on one hand, $t\bar{t}b\bar{b}$ production is an indispensable component in a future global fit for top quark interactions in the SMEFT, and on the other, multi-class machine learning algorithms can be a valuable tool for interpreting LHC data in this framework.

hep-ph↗

arXiv2015

Signatures of top flavour-changing dark matter

We develop the phenomenology of scenarios in which a dark matter candidate interacts with a top quark through flavour-changing couplings, employing a simplified dark matter model with an s-channel vector-like mediator. We study in detail the top-charm flavour-changing interaction, by investigating the single top plus large missing energy signature at the LHC as well as constraints from the relic density and direct and indirect dark matter detection experiments. We present strategies to distinguish between the top-charm and top-up flavour-changing models by taking advantage of the lepton charge asymmetry as well as by using charm-tagging techniques on an extra jet. We also show the complementarity between the LHC and canonical dark matter experiments in exploring the viable parameter space of the models.

hep-ph↗

arXiv2014

Multilepton signals of gauge mediated supersymmetry breaking at the LHC

We investigate multilepton LHC signals arising from electroweak processes involving sleptons. We consider the framework of general gauge mediated supersymmetry breaking, focusing on models where the low mass region of the superpartner spectrum consists of the three generations of charged sleptons and the nearly massless gravitino. We demonstrate how such models can provide an explanation for the anomalous four lepton events recently observed by the CMS collaboration, while satisfying other existing experimental constraints. The best fit to the CMS data is obtained for a selectron/smuon mass of around 145 GeV and a stau mass of around 90 GeV. These models also give rise to final states with more than four leptons, offering alternative channels in which they can be probed and we estimate the corresponding production rates at the LHC.

hep-ph↗

arXiv2013

Automated mass spectrum generation for new physics

We describe an extension of the FeynRules package dedicated to the automatic generation of the mass spectrum associated with any Lagrangian-based quantum field theory. After introducing a simplified way to implement particle mixings, we present a new class of FeynRules functions allowing both for the analytical computation of all the model mass matrices and for the generation of a C++ package, dubbed ASperGe. This program can then be further employed for a numerical evaluation of the rotation matrices necessary to diagonalize the field basis. We illustrate these features in the context of the Two-Higgs-Doublet Model, the Minimal Left-Right Symmetric Standard Model and the Minimal Supersymmetric Standard Model.

hep-ph↗

arXiv2002

Possible reduction of the total uncertainty on the W boson mass measured at LEP2

An alternative W mass estimator in e+e- -> WW -> qqqq events at LEP2 is designed to optimize the balance between the statistical uncertainty and the systematic uncertainty due to a possible Colour Reconnection effect. The preliminary result for the total uncertainty on the W mass in this channel is roughly 30 % lower then those obtained with the standard estimators, based on the SKI implementation of Colour Reconnection. Also an indirect measurement of the SKI Colour Reconnection model parameter kappa is inferred from the difference between both W mass estimators.

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