arXiv subjects

Celine Degrande

Publications and source records attributed to Celine Degrande.

At least 19 recordsLinked to original sources

arXiv2024

Collider sensitivity to SMEFT heavy-quark operators at one-loop in top-quark processes

We study the effects of four-heavy-quark operators in the production of top quarks in the framework of the Standard Model Effective Field Theory (SMEFT) at the LHC. In particular, we compute for the first time the total contribution of the four-top-quark operator which enters only at the one-loop level in the top-quark pair production process. Analytical results at one-loop are presented for the gluon- and quark-initiated sub-processes, which allowed a first complete validation of the SMEFT@NLO in Madgraph5_aMC@NLO. The 95% CL bounds on four-heavy-quark operators from the available top-quark pair and four-top-quark production data are provided, which are complementary to other bounds found in the literature. We focus on the comparison of the sensitivities of the top-quark pair and the four-top-quark production processes, where in the latter case the four-top-quark operator contributes at tree-level. We conclude that the sensitivities of the two processes to four-heavy-quark operators are comparable. The projected sensitivities of both processes at HL-LHC are also presented.

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arXiv2020

Reviving the interference: framework and proof-of-principle for the anomalous gluon self-interaction in the SMEFT

Interferences are not positive-definite and can change sign over the phase space. If the contributions of the regions where they are positive and negative nearly cancel each other, their effects can be hard to measure. In this paper, we propose a method to quantify the ability of an observable to separate these opposite contributions and therefore to revive the interference effects at experiments. We apply this strategy to the anomalous gluon operator in the SMEFT, for which the linear-term suppression is well known. We show that we can get, for the first time, constraints on its coefficient from the interference only that are similar to those from the square of the new-physics amplitude.

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arXiv2019

Can New Physics hide inside the proton?

Modern global analyses of the structure of the proton include collider measurements which probe energies well above the electroweak scale. While these provide powerful constraints on the parton distribution functions (PDFs), they are also sensitive to beyond the Standard Model (BSM) dynamics if these affect the fitted distributions. Here we present a first simultaneous determination of the PDFs and BSM effects from deep-inelastic structure function data by means of the NNPDF framework. We consider representative four-fermion operators from the SM Effective Field Theory (SMEFT), quantify to which extent their effects modify the fitted PDFs, and assess how the resulting bounds on the SMEFT degrees of freedom are modified. Our results demonstrate how BSM effects that might otherwise be reabsorbed into the PDFs can be systematically disentangled.

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arXiv2019

New Physics in Double Higgs Production at Future $e^+ e^-$ Colliders

We study the effects of new physics in double Higgs production at future $e^+ e^-$ colliders. In the Standard Model the chiral limit ($m_e=0$) plays an important role for this process, being responsible for the smallness of the tree-level diagrams with respect to the 1-loop contributions. In our work, we consider the possibility of an enhancement due to the contribution of Standard Model dimension-six effective operators. We show that there are only two relevant operators for this process that are not yet (strongly) constrained by other data. We perform a sensitivity study on the operator coefficients for several benchmark values of energy and integrated luminosity related to the proposed linear colliders such as CLIC, ILC and FCC-ee and we derive expected 95% CL limits for each benchmark scenario.

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arXiv2018

Scalar decays to $γγ$, $Zγ$, and $Wγ$ in the Georgi-Machacek model

We compute the decay widths for the neutral and singly-charged Higgs bosons in the Georgi-Machacek model into the final states $γγ$, $Zγ$, and $Wγ$. These decays are most phenomenologically interesting for the fermiophobic custodial fiveplet states $H_5^0$ and $H_5^{\pm}$ when their masses are below threshold for decays into $WW$, $ZZ$, or $WZ$. We study the allowed branching ratios into these final states using scans over the allowed parameter space, and show how the model can be constrained by LEP searches for a fermiophobic Higgs boson decaying to two photons. The calculation involves evaluating one-loop diagrams in which the loop contains particles with two different masses, some of which do not appear in the existing literature. We give results for these diagrams in a form convenient for numerical implementation using the LoopTools package.

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arXiv2018

Single-top associated production with a $Z$ or $H$ boson at the LHC: the SMEFT interpretation

At the LHC, top quarks can be produced singly with a sizeable rate via electroweak interactions. This process probes a limited set of top-quark electroweak couplings, i.e. , the same entering the top-quark decay, yet at higher scales and with a different sensitivity. Requiring the production of a $Z$ or $H$ boson in association with single-top significantly extends the sensitivity of this process to new physics, opening up the unique possibility of testing top-Higgs, top-gauge, triple gauge, gauge-Higgs interactions without being dominated by QCD interactions. We consider $tZj$ and $tHj$ production at the LHC, providing predictions at next-to-leading accuracy in QCD in the framework of the standard model effective field theory, including all relevant operators up to dimension six. We perform the first complete study of the sensitivity to new interactions of these processes, highlighting the interplay and complementarity among $tj$, $tZj$ and $tHj$ in simultaneously constraining top-quark, triple gauge, and gauge-Higgs interactions in the current and future runs at the LHC.

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arXiv2017

Accurate predictions for charged Higgs production: closing the $m_{H^{\pm}}\sim m_t$ window

We present predictions for the total cross section for the production of a charged Higgs boson in the intermediate-mass range ($m_{H^{\pm}}\sim m_t$) at the LHC, focusing on a type-II two-Higgs-doublet model. Results are obtained at next-to-leading order (NLO) accuracy in QCD perturbation theory, by studying the full process $pp\to H^\pm W^\mp b \bar b$ in the complex-(top)-mass scheme with massive bottom quarks. Compared to lowest-order predictions, NLO corrections have a sizeable impact: they increase the cross section by roughly 50\% and reduce uncertainties due to scale variations by more than a factor of two. Our computation reliably interpolates between the low- and high-mass regime. Our results provide the first NLO prediction for charged Higgs production in the intermediate-mass range and therefore allow to have NLO accurate predictions in the full $m_{H^{\pm}}$ range. The extension of our results to different realisations of the two-Higgs-doublet model or to the supersymmetric case is also discussed.

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arXiv2017

Circular polarisation: a new probe of dark matter and neutrinos in the sky

The study of anomalous electromagnetic emission in the sky is the basis of indirect searches for dark matter. It is also a powerful tool to constrain the radiative decay of active neutrinos. Until now, quantitative analyses have focused on the flux and energy spectrum of such an emission; polarisation has never been considered. Here we show that we could be missing out on an essential piece of information. The radiative decay of neutrinos, as well as the interactions of dark matter and neutrinos with Standard Model particles can generate a circular polarisation signal in X-rays or γ-rays. If observed, this could reveal important information about their spatial distribution and particle-antiparticle ratio, and could even reveal the nature of the high-energy particle physics processes taking place in astrophysical sites. The question of the observability of these polarised signatures and their separation from background astrophysical sources is left for future work.

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arXiv2017

Electroweak Higgs boson production in the standard model effective field theory beyond leading order in QCD

We study the impact of dimension-six operators of the standard model effective field theory relevant for vector-boson fusion and associated Higgs boson production at the LHC. We present predictions at the next-to-leading order accuracy in QCD that include matching to parton showers and that rely on fully automated simulations. We show the importance of the subsequent reduction of the theoretical uncertainties in improving the possible discrimination between effective field theory and standard model results, and we demonstrate that the range of the Wilson coefficient values allowed by a global fit to LEP and LHC Run~I data can be further constrained by LHC Run~II future results.

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arXiv2017

NLO predictions for the production of a spin-two particle at the LHC

We obtain predictions accurate at the next-to-leading order in QCD for the production of a generic spin-two particle in the most relevant channels at the LHC: production in association with coloured particles (inclusive, one jet, two jets and $t\bar t$), with vector bosons ($Z,W^\pm,γ$) and with the Higgs boson. We present total and differential cross sections as well as branching ratios as a function of the mass and the collision energy also considering the case of non-universal couplings to standard model particles. We find that the next-to-leading order corrections give rise to sizeable $K$ factors for many channels, in some cases exposing the unitarity-violating behaviour of non-universal couplings scenarios, and in general greatly reduce the theoretical uncertainties. Our predictions are publicly available in the MadGraph5\_aMC@NLO framework and can, therefore, be directly used in experimental simulations of spin-two particle production for arbitrary values of the mass and couplings.

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arXiv2016

Multi-Higgs production in gluon fusion at 100 TeV

We carry out a detailed study of multi-Higgs production processes in the gluon fusion channel in the high energy regime relevant to Future Circular hadron colliders and in the high-Higgs-multiplicity limit (> 20). Our results are based on the computation of the leading polygons - the triangles, boxes, pentagons and hexagons - to the scattering processes, further combined with the subsequent branchings to reach high final state multiplicities. The factorial growth of the number of diagrams leads to an exponential enhancement of such large multiplicity cross-sections and, ultimately, in breaking of perturbativity. We find that the characteristic energy and multiplicity scales where these perturbative rates become highly enhanced and grow with increasing energy are within the 100 TeV regime with of the order of 130 Higgses (or more) in the final state. We also show that already for a 50 TeV hadron collider the perturbative cross-sections for 140 bosons are at picobarn level.

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arXiv2016

Fully-Automated Precision Predictions for Heavy Neutrino Production Mechanisms at Hadron Colliders

Motivated by TeV-scale neutrino mass models, we propose a systematic treatment of heavy neutrino $(N)$ production at hadron colliders. Our simple and efficient modeling of the vector boson fusion (VBF) $W^\pmγ\rightarrow N\ell^\pm$ and $N\ell^\pm+nj$ signal definitions resolve collinear and soft divergences that have plagued past studies, and is applicable to other color-singlet processes, e.g., associated Higgs $(W^\pm h)$, sparticle $(\tilde{\ell}^\pm\tilde{ν_\ell})$, and charged Higgs $(h^{\pm\pm}h^{\mp})$ production. We present, for the first time, a comparison of all leading $N$ production modes, including both gluon fusion (GF) $gg\rightarrow Z^*/h^*\rightarrow N\overset{(-)}{ν_\ell}$ and VBF. We obtain fully differential results up to next-to-leading order (NLO) in QCD accuracy using a Monte Carlo tool chain linking FeynRules, N{\small LO}CT, and MadGraph5\_aMC@NLO. Associated model files are publicly available. At the 14 TeV LHC, the leading order GF rate is small and comparable to the NLO $N\ell^\pm+1j$ rate; at a future 100 TeV Very Large Hadron Collider, GF dominates for $m_N={300-1500}$ GeV, beyond which VBF takes lead.

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arXiv2016

A basis of dimension-eight operators for anomalous neutral triple gauge boson interactions

Four independent dimension-eight operators give rise to anomalous neutral triple gauge boson interactions, one CP-even and three CP-odd. Only the CP-even operator interferes with the Standard Model for the production of a pair of on-shell neutral bosons. However, the effects are found to be tiny due mainly to the mismatch of the Z boson polarization between the productions from the SM and the new operator.

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arXiv2016

Matching next-to-leading order predictions to parton showers in supersymmetric QCD

We present a fully automated framework based on the FeynRules and MadGraph5 aMC@NLO programs that allows for accurate simulations of supersymmetric QCD processes at the LHC. Starting directly from a model Lagrangian that features squark and gluino interactions, event generation is achieved at the next-to-leading order in QCD, matching short-distance events to parton showers and including the subsequent decay of the produced supersymmetric particles. As an application, we study the impact of higher-order corrections in gluino pair-production in a simplified benchmark scenario inspired by current gluino LHC searches.

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arXiv2015

Automatic predictions in the Georgi-Machacek model at next-to-leading order accuracy

We study the phenomenology of the Georgi-Machacek model at next-to-leading order (NLO) in QCD matched to parton shower, using a fully-automated tool chain based on MadGraph5_aMC@NLO and FeynRules. We focus on the production of the fermiophobic custodial fiveplet scalars H_5^0, H_5^+/-, and H_5^++/-- through vector boson fusion (VBF), associated production with a vector boson (V H_5), and scalar pair production (H_5 H_5). For these production mechanisms we compute NLO corrections to production rates as well as to differential distributions. Our results demonstrate that the Standard Model (SM) overall K-factors for such processes cannot in general be directly applied to beyond-the-SM distributions, due both to differences in the scalar electroweak charges and to variation of the K-factors over the differential distributions.

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arXiv2015

Heavy charged Higgs boson production at the LHC

In this paper we study the production of a heavy charged Higgs boson in association with heavy quarks at the LHC, in a type-II two-Higgs-doublet model. We present for the first time fully-differential results obtained in the four-flavour scheme at NLO accuracy, both at fixed order and including the matching with parton showers. Relevant differential distributions are studied for two values of the charged boson mass and a thorough comparison is performed between predictions obtained in the four- and five-flavour schemes. We show that the agreement between the two schemes is improved by NLO(+PS) corrections for observables inclusive in the degrees of freedom of bottom quarks. We argue that the four-flavour scheme leads to more reliable predictions, thanks to its accurate description of the bottom-quark kinematics and its small dependence on the Monte Carlos, which in turn is rather large in the five-flavour scheme. A detailed set of recommendations for the simulation of this process in experimental analyses at the LHC is provided.

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arXiv2015

Automatic computations at next-to-leading order in QCD for top-quark flavor-changing neutral processes

Computations at next-to-leading order in the Standard Model offer new technical challenges in presence of higher dimensional operators. We introduce a framework that, starting from the top-quark effective field theory at dimension six, allows to make predictions for cross sections as well as distributions in a fully automatic way. As an application, we present the first complete results at NLO in QCD for flavor changing neutral interactions including parton shower effects, for $tZ$, $th$, $tγ$ associated production at the LHC.

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arXiv2014

Automated Two Higgs Doublet Model at NLO

The Two Higgs Doublet Model at NLO is generated automatically by FeynRules and NLOCT and allows any computation to be performed at NLO in QCD inside MadGraph5_aMC@NLO. The model can handle both four and five massless flavours. Preliminary results of the shape comparison between the two schemes are shown.

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