arXiv subjects

Céline Degrande

Publications and source records attributed to Céline Degrande.

17 recordsLinked to original sources

arXiv2026

Constraining the four-light quark operators in the SMEFT with multijet and VBF processes at linear level

We investigate how the interference of the SM with ten four-light quark operators in the SMEFT can be constrained thanks to multijet and $Z$, $W$, $γ$ VBF production in association with jets. The differential distributions for each process are generated at LO for different jet multiplicities, that are then merged and showered. We check which observables provide better bounds on the Wilson coefficients, and what directions in the ten-dimensional coefficient space they are able to probe. We discuss the relevance of the quadratic contributions with respect to the linear terms and use them to assess the validity of the EFT approach.

hep-ph↗

arXiv2026

Partial-Wave Unitarity Bounds on Higher-Dimensional Operators from 2-to-$N$ Scattering

We present a systematic method for deriving partial-wave unitarity bounds on Wilson coefficients of higher-dimensional operators in effective field theories involving more than four fields, which naturally appear in tree-level 2-to-$N$ scattering processes with $N \geq 3$. Unlike 2-to-2 scattering, 2-to-$N$ scattering with $N \geq 3$ features multiple amplitudes associated with the same total angular momentum. To resolve these degeneracies, we provide a way to construct an orthonormal amplitude basis by parameterizing the phase space manifold of massless particles using spinor-helicity variables, enabling analytical integration over the phase space with arbitrary particle numbers. We provide Mathematica code to analytically evaluate phase space integrals of interference between two local on-shell amplitudes up to four final-state particles, with straightforward generalization to $N$ final-state particles. As practical applications, we demonstrate the use of this tool by deriving unitarity bounds on some dimension-7 and dimension-8 operators in the Standard Model effective field theory involving five and six fields, respectively.

hep-ph↗

arXiv2025

EFT observable stability under NLO corrections through interference revival

We illustrate the importance of interference revival, when higher-order corrections are included, by presenting LO and NLO differential cross sections and $K$-factors for three processes that are sensitive to the dimension-6 SMEFT operator $O_W$ : $Z$-plus-two-jets ($Zjj$) through VBF, leptonic diboson $WZ$ and $Wγ$ production. We show how lifting the interference suppression at LO, through suitable variables and cuts, is necessary to get reliable predictions at NLO. We also present bounds on $C_W /Λ^2$ obtained from these observables

hep-ph↗

arXiv2023

UFO 2.0 -- The Universal Feynman Output format

We present an update of the Universal FeynRules Output model format, commonly known as the UFO format, that is used by several automated matrix-element generators and high-energy physics software. We detail different features that have been proposed as extensions of the initial format during the last ten years, and collect them in the current second version of the model format that we coin the Universal Feynman Output format. Following the initial philosophy of the UFO, they consist of flexible and modular additions to address particle decays, custom propagators, form factors, the renormalisation group running of parameters and masses, and higher-order quantum corrections.

hep-ph↗

arXiv2023

Impact of Dimension-8 SMEFT operators on Diboson Productions

We for the first time identify all the dimension-8 (dim-8) SMEFT operators that can have an interference with the SM with $E^4/Λ^4$ enhancement in the high energy limit for the processes $q\bar{q}\to WW/WZ$. Our results therefore explicitly show that the non-interference observed for the dimension-six does not extend to dimension-eight. We compute the contributions of those dimension-8 operators to the cross-section at the 14 TeV Large Hadron Collider and compare the results with dimension-6 (dim-6) originated corrections at the order of dim-6-SM interference and dim-6 squared. We find one (two) dim-8 operator(s) can generate amplitudes of a similar order of magnitude compared with their dim-6 squared counterparts assuming unity dimensionless Wilson coefficients. During the study, new non-interference scenarios are found due to the selection rule of angular momentum as well as strong suppression due to the symmetric initial state for the proton-proton collider.

hep-ph↗

arXiv2022

Truncation, validity, uncertainties

The truncation of the standard-model effective field theory, its validity and the associated uncertainties have been discussed in meetings of the LHC EFT WG. Proposals were made by participants to address these issues. No consensus was reached and no formal recommendation is therefore put forward at this time. None of the proposals has been approved or validated and further work is needed to establish a prescription. This note aims at summarizing the proposals and points of debate.

hep-ph↗

arXiv2022

Signatures of leptophilic t-channel dark matter from active galactic nuclei

In this work, we study indirect photon signatures of leptophilic dark matter (DM) coming from Centaurus A, where a DM density spike is believed to have survived to date contrary to the case of our galaxy. We consider a model where DM is a Majorana fermion which interacts with right-handed electrons via a scalar mediator. Assuming that the photons measured from the core of Cen A are coming from SM processes, we derive constraints on the average annihilation cross section which are 7 orders of magnitude stronger than the ones from measurements of the Galactic Center. Focusing on the allowed parameter space range, we calculate the flux of photons coming from the radiative DM-electron scattering in the AGN jets and the circular polarisation asymmetry of these photons. We find that this flux is two orders of magnitude lower than the background but its circular polarisation asymmetry can reach values close to $100\%$, indicating the need to experimentally exploit the high fraction of circular polarisation in order to detect these interactions. Since the origin of the photons in the GeV-TeV range from Cen A is not completely clear and an exotic origin is compatible with the measurements as well, we also consider the scenario in which synchrotron radiation can only partially explain the photon flux and we fit the excess with signals coming from DM annihilation, finding a best fit for a DM candidate with a mass $m_{\tildeχ}= 123$ GeV and a coupling $a_R = 0.018$.

hep-ph↗

arXiv2021

Circular polarisation of gamma rays as a probe of dark matter interactions with cosmic ray electrons

Conventional indirect dark matter (DM) searches look for an excess in the electromagnetic emission from the sky that cannot be attributed to known astrophysical sources. Here, we argue that the photon polarisation is an important feature to understand new physics interactions and can be exploited to improve our sensitivity to DM. In particular, circular polarisation can be generated from Beyond the Standard Model interactions if they violate parity and there is an asymmetry in the number of particles which participate in the interaction. In this work, we consider a simplified model for fermionic (Majorana) DM and study the circularly polarised gamma rays below 10 GeV from the scattering of cosmic ray electrons on DM. We calculate the differential flux of positive and negative polarised photons from the Galactic Center and show that the degree of circular polarization can reach up to 90%. Finally, once collider and DM constraints have been taken into account, we estimate the required sensitivity from future experiments to detect this signal finding that, although a distinctive peak will be present in the photon flux spectrum, a near future observation is unlikely. However, different sources or models not considered in this work could provide higher intensity fluxes, leading to a possible detection by e-ASTROGAM. In the event of a discovery, we argue that the polarisation fraction is a valuable characterisation feature of the new sector.

hep-ph↗

arXiv2021

Searching for Dark Matter with circularly polarised photons from the Galactic Centre

Conventional indirect dark matter (DM) searches look for an excess in the electromagnetic emission from the sky that cannot be attributed to known astrophysical sources, but its polarisation has not been explored to date. In this proceeding, we argue that photon polarisation is an important feature to understand new physics interactions. In particular, circular polarisation can be generated from Beyond the Standard Model interactions if they violate parity and there is an asymmetry in the number density of the initial state particles which participate in the interaction. We consider a simplified model for fermionic (Majorana) DM and study the circularly polarised gamma rays below 10 GeV from DM cosmic ray electron interactions. We study the differential flux of positive and negative polarised photons from the Galactic Centre and show that the degree of circular polarization can reach up to 90%. Finally, we discuss the detection prospects of this signal in future experiments.

hep-ph↗

arXiv2021

A Reduced basis for CP violation in SMEFT at colliders and its application to Diboson production

We show that only 10 (17) CP-odd operators of the SMEFT give the leading, i.e. least suppressed by the new physics scale, CP-violating contributions once we assume that all fermions are massless but the top (and bottom) quark(s). We start with a short review of previous analyses focusing on operators of our reduced basis and list different observables probing their CP violating effects by direct measurements at colliders and by indirect measurements in low-energy observables. Since CP-odd operators typically lead to phase space suppressed interferences, we quantify the efficiency to revive the interference for various observables found in the literature but also for new observables in diboson production. Our new observables are found to be more efficient on the whole experimental fiducial phase space and are complementary to those presented so far as they probe different combinations of operators and get their sensitivities from different regions of the phase space.

hep-ph↗

arXiv2021

Automated one-loop computations in the SMEFT

We present the automation of one-loop computations in the standard-model effective field theory at dimension six. Our general implementation, dubbed SMEFT@NLO, covers all types of operators: bosonic, two- and four-fermion ones. Included ultraviolet and rational counterterms presently allow for fully differential predictions, possibly matched to parton shower, up to the one-loop level in the strong coupling or in four-quark operator coefficients. Exact flavor symmetries are imposed among light quark generations and an initial focus is set on top-quark interactions in the fermionic sector. We illustrate the potential of this implementation with novel loop-induced and next-to-leading-order computations relevant for top-quark, electroweak, and Higgs-boson phenomenology at the LHC and future colliders.

hep-ph↗

arXiv2020

Using Circular Polarisation to Test the Composition and Dynamics of Astrophysical Particle Accelerators

We investigate the production of circularly polarised X and gamma-ray signals in cosmic accelerators such as supernova remnants and AGN jets. Proton-proton and proton-photon collisions within these sites produce a charge asymmetry in the distribution of mesons and muons that eventually leads to a net circular polarisation signal as these particles decay radiatively. We find that the fraction of circular polarisation thus produced is at the level of $ 5 \times 10^{-4}$, regardless of the exact beam spectrum, as long as it is made predominantly of protons. While this fraction is very small, the detection of circular polarisation signals in conjunction with high-energy neutrinos would provide an unambiguous signature of the presence of high-energy protons in cosmic accelerators. In Supernovae shocks in particular, this would indicate the presence of relativistic protons hitting stationary protons and/or low-energy photons in the intergalactic or interstellar medium.

astro-ph.HE↗

arXiv2020

Global fit of pseudo-Nambu-Goldstone Dark Matter

We perform a global fit within the pseudo-Nambu-Goldstone dark matter (DM) model emerging from an additional complex scalar singlet with a softly broken global U(1) symmetry. Leading to a momentum-suppressed DM-nucleon cross section at tree level, the model provides a natural explanation for the null results from direct detection experiments. Our global fit combines constraints from perturbative unitarity, DM relic abundance, Higgs invisible decay, electroweak precision observables and latest Higgs searches at colliders. The results are presented in both frequentist and Bayesian statistical frameworks. Furthermore, post-processing our samples, we include the likelihood from gamma-ray observations of $\mathit{Fermi}$-LAT dwarf spheroidal galaxies and compute the one-loop DM-nucleon cross section. We find two favoured regions characterised by their dominant annihilation channel: the Higgs funnel and annihilation into Higgs pairs. Both are compatible with current $\mathit{Fermi}$-LAT observations, and furthermore, can fit the slight excess observed in four dwarfs in a mass range between about 30-300 GeV. While the former region is hard to probe experimentally, the latter can partly be tested by current observations of cosmic-ray antiprotons as well as future gamma-ray observations.

hep-ph↗

arXiv2019

Proposal for the validation of Monte Carlo implementations of the standard model effective field theory

We propose a procedure to cross-validate Monte Carlo implementations of the standard model effective field theory. It is based on the numerical comparison of squared amplitudes computed at specific phase-space and parameter points in pairs of implementations. Interactions are fully linearised in the effective field theory expansion. The squares of linear effective field theory amplitudes and their interference with standard-model contributions are compared separately. Such pairwise comparisons are primarily performed at tree level and a possible extension to the one-loop level is also briefly considered. We list the current standard model effective field theory implementations and the comparisons performed to date.

hep-ph↗

arXiv2015

Automated next-to-leading order predictions for new physics at the LHC: the case of colored scalar pair production

We present for the first time the full automation of collider predictions matched with parton showers at the next-to-leading accuracy in QCD within non-trivial extensions of the Standard Model. The sole inputs required from the user are the model Lagrangian and the process of interest. As an application of the above, we explore scenarios beyond the Standard Model where new colored scalar particles can be pair produced in hadron collisions. Using simplified models to describe the new field interactions with the Standard Model, we present precision predictions for the LHC within the MadGraph5 aMC@NLO framework.

hep-ph↗

arXiv2012

UFO - The Universal FeynRules Output

We present a new model format for automatized matrix-element generators, the so- called Universal FeynRules Output (UFO). The format is universal in the sense that it features compatibility with more than one single generator and is designed to be flexible, modular and agnostic of any assumption such as the number of particles or the color and Lorentz structures appearing in the interaction vertices. Unlike other model formats where text files need to be parsed, the information on the model is encoded into a Python module that can easily be linked to other computer codes. We then describe an interface for the Mathematica package FeynRules that allows for an automatic output of models in the UFO format.

hep-ph↗

arXiv2011

Non-resonant New Physics in Top Pair Production at Hadron Colliders

We use top quark pair production as a probe of top-philic non-resonant new physics. Following a low energy effective field theory approach, we calculate several key observables in top quark pair production at hadron colliders (e.g., total cross section, ttbar invariant mass distribution, forward-backward asymmetry, spin correlations) including the interference of the Standard Model with dimension-six operators. We determine the LHC reach in probing new physics after having taken into account the Tevatron constraints. In particular, we show that the gluon fusion process gg -> ttbar which remains largely unconstrained at the Tevatron is affected by only one top-philic dimension-six operator, the chromo-magnetic moment of the top quark. This operator can be further constrained by the LHC data as soon as a precision of about 20% is reached for the total ttbar cross-section. While our approach is general and model-independent, it is particularly relevant to models of Higgs and top compositeness, which we consider in detail, also in connection with ttbar ttbar and ttbar bbbar production.

hep-ph↗