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António Onofre

Publications and source records attributed to António Onofre.

At least 19 recordsLinked to original sources
arXiv2026

$t \bar{t}$ production as a window to invisible new physics

We present a phenomenological study where we probe the sensitivity to invisible dark matter (DM) mediators produced in association with a $t\bar{t}$ pair at the Large Hadron Collider (LHC). Building on previous work focused on scalar mediators, we extend the analysis to include spin-1 mediators, $Y_1$, with both vector and axial-vector couplings to top quarks. The mediator mass is fixed to 5 GeV. Signal samples of $pp \rightarrow t\bar{t}Y_i$ ($i = 0, 1$) are generated using a MadGraph5_aMC@NLO simplified DM model. Only dileptonic final states of the $t\bar{t}$ system are considered, and the reconstruction is performed through a kinematic fit without explicitly reconstructing the invisible mediator. All relevant Standard Model backgrounds are included. We consider several exclusion scenarios to assess the sensitivity to the presence of a spin-1 mediator, as well as the ability to distinguish a pure vector or axial-vector mediator from alternative hypotheses with different spin and CP properties. We find that the analysis is sensitive to light spin-1 mediators and that CP-sensitive angular observables provide discrimination power between vector, axial-vector, scalar and pseudoscalar scenarios. These results highlight the potential of $t\bar{t}$ final states not only to search for invisible particles, but also to characterize their spin and parity properties in case of discovery.

hep-ph↗
arXiv2026

Extracting a Toponium Signal at the LHC with Spin and Quantum Information Tools

We investigate near-threshold top-antitop production at the LHC, focusing on the impact of toponium formation on spin correlations and quantum information properties of the final state. Considering the top-antitop system as a mixed two-qubit state, we reconstruct spin density matrices via quantum tomography and evaluate several observables including some inspired by quantum information. We then compare their sensitivity in discriminating toponium effects from top-antitop production without these effects. Our results demonstrate that combining these variables is expected to significantly enhance sensitivity to toponium effects, bringing new ways to explore these subtle features.

hep-ph↗
arXiv2025

Observing boson stars in binary systems: The case of Gaia BH1

The Gaia experiment recently reported the observation of a binary system composed of a Sun-like star orbiting a dark compact object, known as Gaia BH1. The nature of the compact object remains uncertain. While the Gaia mission identifies it as a black hole candidate, the absence of X-ray or radio detections challenges that interpretation, and alternative exotic compact objects such as boson stars have also been suggested. In this paper, we study whether a boson star could account for the observed properties of the source. To do so we compute the X-ray luminosity of the central dark object as a result of spherically symmetric (Bondi-Michel) accretion of matter, comparing our results for the cases in which the dark object is a Schwarzschild black hole or a non rotating boson star. Our model incorporates realistic interstellar medium properties, ranging from hot ionized gas to dense molecular clouds. By solving the governing equations numerically, we calculate mass accretion rates and derive the resulting Bremsstrahlung X-ray luminosities. Black holes and boson stars fundamentally differ by the absence of an event horizon in the latter, which directly impacts accretion dynamics as there is an accumulation of mass in regions closer to the boson star, which will significantly change the observed X-ray emission. For the Gaia BH1 system we find that accretion onto a black hole yields luminosities of $\sim10^{27} \ \text{erg}\, \text{cm}^{-2}\, \text{s}^{-1}$ which corresponds to an X-ray flux undetectable by Chandra sensitivity. On the other hand, boson star accretion can produce observable luminosities in the order of $10^{27} \ \text{to} \ 10^{41} \ \text{erg}\, \text{cm}^{-2}\, \text{s}^{-1}$.

astro-ph.HE↗
arXiv2024

Asymmetries in invisible Dark Matter mediator production associated with $t \bar{t}$ final states

In this paper, we propose two sets of different CP-sensitive observables inspired by the Higgs production in association with the top quark. We employ a Dark Matter simplified model that couples a scalar mediator with top quarks. The reconstruction of the kinematic variables is presented at NLO accuracy for events associated with this massive scalar particle, which is assumed to be vanishing to invisible decays in a detector such as ATLAS. We build these observables by taking advantage of the similarity between the scalar coupling with the top quark and the factorization theorem in the total scattering amplitude, in order to represent the basis in which the phase space is parameterized. A twofold approach employs the direct implementation of the four-momentum phase space measure in building CP sensitive observables such as $b_{2}$ for the Higgs, and the spin polarization of the top-quark decays in the narrow width approximation for the employed model. We studied the asymmetries of these distributions to test for any improvement in increasing the exclusion region for the $g_{u_{33}}^S-g_{u_{33}}^P$ parameters associated with this vanishing scalar particle. We have found no significant effect in the exclusion limits by using the forward-backward asymmetry distributions and the full-shaped ones. Considering the case of an invisible mediator with mass of 10$^{-2}$ GeV for a luminosity of 300 fb$^{-1}$ expected at the end of Run 3, the best limits for $g_{u_{33}}^S$ and $g_{u_{33}}^P$ at NLO accuracy were obtained using the variables ${\tilde{b}}_{2}^{\widehat{y}}$ and $b_{2}$ respectively, with corresponding limits set to $[-0.0425, 0.0425] $ and $[-0.83, 0.83]$ at $68\%$ CL.

hep-ph↗
arXiv2023

Exploring mixed lepton-quark interactions in non-resonant leptoquark production at the LHC

Searches for new physics (NP) at particle colliders typically involve multivariate analysis of kinematic distributions of final state particles produced in a decay of a hypothetical NP resonance. Since the pair-production cross-sections mediated by such resonances are strongly suppressed by the NP scale, this analysis becomes less relevant for NP searches for masses of the BSM resonance above 1 TeV. On the other hand, $t$-channel processes are less sensitive to the mass of the virtual mediator and therefore larger phase-space can be potentially probed as well as the couplings between the NP particles and the Standard Model fields. The fact that transitions between different generations of quarks and leptons may exist, the potential of the search presented in this article can be used, as a reference guide, to enlarge significantly the scope of searches performed at the LHC to flavour off-diagonal channels, in a theoretically consistent approach. In this work, we study non-resonant production of scalar leptoquarks which have been proposed in the literature to provide a potential avenue for radiative generation of neutrino masses, accommodating as well the existing flavour physics data. Final states involving just two muons at the LHC ($μ^+, μ^-$), are used as a well-motivated case study.

hep-ph↗
arXiv2023

Search for an invisible scalar in $t \bar{t}$ final states at the LHC

We use the current $t\bar t$ experimental analysis to look for Dark Matter (DM) particles hidden in the final state. We present a phenomenological study where we successfully perform the reconstruction of a $t\bar{t}$ system in the presence of a scalar mediator $Y_0$, that couples to both Standard Model (SM) and to DM particles. We use a \texttt{MadGraph5\_aMC@NLO} simplified DM model, where signal samples of $pp \rightarrow t\bar{t}Y_0$ are generated at the Large Hadron Collider (LHC) with both Charge-Parity (CP) -even and CP-odd couplings of $Y_0$ to the top quarks. Different mass scales for the $Y_0$ mediator are considered, from the low mass region ($\sim$ 0~GeV) to masses close to the Higgs boson mass (125~GeV). The dileptonic final states of the $t\bar{t}$ system were used in our analysis. The reconstruction of the $t\bar{t}$ system is done with a kinematic fit, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic $t\bar{t}$ search at the LHC are considered. Furthermore, CP angular observables were used to probe the CP-nature of the coupling between the mediator and top-quarks, which allowed to set confidence level (CL) limits for those Yukawa couplings as a function of the mediator mass.

hep-ph↗
arXiv2023

Collider phenomenology of new neutral scalars in a flavoured multi-Higgs model

In this work, we propose and explore for the first time a new collider signature of heavy neutral scalars typically found in many distinct classes of multi-Higgs models. This signature, particular relevant in the context of the Large Hadron Collider (LHC) measurements, is based on a topology with two charged leptons and four jets arising from first and second generation quarks. As an important benchmark scenario of the multi-Higgs models, we focus on a recently proposed Branco-Grimus-Lavoura (BGL) type model enhanced with an abelian U(1) flavour symmetry and featuring an additional sector of right-handed neutrinos. We discuss how kinematics of the scalar fields in this model can be used to efficiently separate the signal from the dominant backgrounds and explore the discovery potential of the new heavy scalars in the forthcoming LHC runs. The proposed method can be applied for analysis of statistical significance of heavy scalars' production at the LHC and future colliders in any multi-Higgs model.

hep-ph↗
arXiv2023

Exploring the viability of pseudo Nambu-Goldstone boson as ultralight dark matter in a mass range relevant for strong gravity applications

We study a simple extension of the Standard Model featuring a dark sector with an ultralight pseudo Nambu-Goldstone boson as dark matter candidate. We focus on the mass range $\mathcal{O}(10^{-20} - 10^{-10})$ eV, relevant for strong gravity applications, and explore its production and evolution in the early Universe. The model is formulated in such a way that dark matter does not couple directly to photons or other Standard Model particles avoiding some of the most stringent cosmological bounds related to axion-like particles. In this work, two different scenarios are considered depending on whether dark matter is produced in a pre-inflationary or post-inflationary regime. We also discuss the effect from emergent topological defects such as cosmic strings and domain walls, and estimate the spectrum of stochastic gravitational waves produced by their decay, enabling to test the model at current and future gravitational-wave experiments.

hep-ph↗
arXiv2023

Deep Learning Searches for Vector-Like Leptons at the LHC and Electron/Muon Colliders

The discovery potential of both singlet and doublet vector-like leptons (VLLs) at the Large Hadron Collider (LHC) as well as at the not-so-far future muon and electron machines is explored. The focus is on a single production channel for LHC direct searches while double production signatures are proposed for the leptonic colliders. A Deep Learning algorithm to determine the discovery (or exclusion) statistical significance at the LHC is employed. While doublet VLLs can be probed up to masses of 1 TeV, their singlet counterparts have very low cross sections and can hardly be tested beyond a few hundreds of GeV at the LHC. This motivates a physics-case analysis in the context of leptonic colliders where one obtains larger cross sections in VLL double production channels, allowing to probe higher mass regimes otherwise inaccessible even to the LHC high-luminosity upgrade.

hep-ph↗
arXiv2022

CP-violation, Asymmetries and Interferences in $t \bar{t} ϕ$

We use the associated production of top-quark pairs with a generic scalar boson at the LHC to explore the sensitivity of a large set of observables to the sign of the CP mixing angle, present in the coupling between the scalar boson and the top quarks. The mass of the scalar boson is set to $m_ϕ=125$ GeV and its coupling to top-quarks is varied such that $α=$ 0$^\circ$, 22.5$^\circ$, 45.0$^\circ$, 67.5$^\circ$, 90.0$^\circ$, 135.0$^\circ$ and 180.0$^\circ$. Dileptonic final states of the $t\bar{t}ϕ$ system are used, with the scalar boson decaying as $ϕ\rightarrow b\bar{b}$. A new method to reconstruct the scalar mass, originally designed for the low mass regime is used, improving the resolution of the Higgs mass by roughly a factor of two. A full phenomenological analysis is performed using Standard Model background and signal events generated with MadGraph5aMC@NLO and reconstructed using a kinematical fit. The most sensitive CP-observables are selected to compute Confidence Level (CL) limits as a function of the sign of the top quark Yukawa couplings to the scalar boson. We also explore the sensitivity to interference terms using differential distributions and angular asymmetries. Given the significant difference between the pure scalar and pure pseudo-scalar production cross section values, it is unlikely the $t\bar{t}ϕ$ channel alone will be sensitive to the sign of the CP-mixing angle or interference terms, even at the end of the LHC. Using the $b_2^{t\bar{t}ϕ}$ and $b_4^{t\bar{t}ϕ}$ variables, exclusion limits at 95\% CL for the CP-even and CP-odd components of the top quark Yukawa couplings are expected to be set to $\tildeκ \in$ [-0.698,+0.698] and $|κ| \in$ [0.878,1.04], respectively, at the end of the High Luminosity phase of the LHC (HL-LHC) by using the dileptonic decay channel alone.

hep-ph↗
arXiv2022

Generating gravitational waveform libraries of exotic compact binaries with deep learning

Current gravitational wave (GW) detections rely on the existence of libraries of theoretical waveforms. Consequently, finding new physics with GWs requires libraries of non-standard models, which are computationally demanding. We discuss how deep learning frameworks can be used to generate new waveforms "learned" from a simulation dataset obtained, say, from numerical relativity simulations. Concretely, we use the WaveGAN architecture of a generative adversarial network (GAN). As a proof of concept we provide this neural network (NN) with a sample of ($>500$) waveforms from the collisions of exotic compact objects (Proca stars), obtained from numerical relativity simulations. Dividing the sample into a training and a validation set, we show that after a sufficiently large number of training epochs the NN can produce from 12\% to 25\% of the synthetic waveforms with an overlapping match of at least 95\% with the ones from the validation set. We also demonstrate that a NN can be used to predict the overlapping match score, with 90\% of accuracy, of new synthetic samples. These are encouraging results for using GANs for data augmentation and interpolation in the context of GWs, to cover the full parameter space of, say, exotic compact binaries, without the need of intensive numerical relativity simulations.

gr-qc↗
arXiv2021

Ultralight bosons for strong gravity applications from simple Standard Model extensions

We construct families, and concrete examples, of simple extensions of the Standard Model that can yield ultralight {real or} complex vectors or scalars with potential astrophysical relevance. Specifically, the mass range for these putative fundamental bosons ($\sim 10^{-10}-10^{-20}$ eV) would lead dynamically to both new non-black hole compact objects (bosonic stars) and new non-Kerr black holes, with masses of $\sim M_\odot$ to $\sim 10^{10} M_\odot$, corresponding to the mass range of astrophysical black hole candidates (from stellar mass to supermassive). For each model, we study the properties of the mass spectrum and interactions after spontaneous symmetry breaking, discuss its theoretical viability and caveats, as well as some of its potential and most relevant phenomenological implications {linking them to the} physics of compact objects.

hep-ph↗
arXiv2020

Light Higgs searches in $t \bar t ϕ$ production at the LHC

In this paper we propose a new reconstruction method to explore the low mass region in the associated production of top-quark pairs ($t\bar{t}$) with a generic scalar boson ($ϕ$) at the LHC. The new method of mass reconstruction shows an improved resolution of at least a factor of two in the low mass region when compared to previous methods, without the loss of sensitivity of previous analyses. It turns out that it also leads to an improvement of the mass reconstruction of the 125 GeV Higgs for the same production process. We use an effective Lagrangian to describe a scalar with a generic Yukawa coupling to the top quarks. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5\_aMC@NLO and reconstructed using a kinematic fit. The use of CP-sensitive variables allows then to maximize the distinction between CP-even and CP-odd components of the Yukawa couplings. Confidence Levels (CLs) for the exclusion of $ϕ$ bosons with mixed CP (both CP-even and CP-odd components) were determined as a function of the top Yukawa couplings to the $ϕ$ boson. The mass range analysed starts slightly above the $Υ$ mass up to 40 GeV, although the analysis can be used for an arbitrary mass. %We focus on dileptonic final states of the $t\bar{t}ϕ$ system, with $ϕ\rightarrow b\bar{b}$. If no new light scalar is found, exclusion limits at 95\% CL for the absolute value of the CP-even and CP-odd Yukawa are derived. %couplings are expected to be, approximately, as low as 0.10 and 0.50, respectively, at the end of the High Luminosity phase of the LHC (HL-LHC). Finally, we analyse how these limits constrain the parameter space of the complex two-Higgs doublet model (C2HDM).

hep-ph↗
arXiv2020

Scalar mass dependence of angular variables in $t\bar tϕ$ production

In this paper we explore CP discrimination in the associated production of top-quark pairs ($t\bar{t}$) with a generic scalar boson ($ϕ$) at the LHC. We probe the CP-sensitivity of several observables for a varying scalar boson mass and CP-number, either CP-even ($ϕ=H$) or CP-odd ($ϕ=A$), using dileptonic final states of the $t\bar{t}ϕ$ system, with $ϕ\rightarrow b\bar{b}$. We show that CP-searches are virtually impossible for $ϕ$ boson masses above a few hundred GeV in this channel. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5 MC@NLO and reconstructed using a kinematic fit. The most sensitive CP-observables are used to compute Confidence Levels (CLs), as a function of luminosity, for the exclusion of different signal hypotheses with scalar and pseudoscalar boson masses that range from $m_ϕ$ = 40 GeV up to 200 GeV. We finalize by analysing the impact of a measurement (or limit) of the CP-violating angle in the parameter space of a complex two-Higgs doublet model known as the C2HDM.

hep-ph↗
arXiv2019

The Role of the $t{\bar t}h$ Rest Frame in Direct Top-Quark Yukawa Coupling Measurements

This paper studies new possibilities to directly measure a hypothetical $CP$-odd (pseudoscalar) component in the top-quark Yukawa coupling. In particular, the role of the $t{\bar t} h$ center-of-mass rest frame in the associated production of a top pair and a $h$ boson at the LHC is explored. The $h$ boson is assumed to have both a $CP$-even (scalar) and a $CP$-odd coupling to the top quark. The relative strength of the scalar and pseudoscalar components is regulated by an angle $α$. Observables sensitive to the nature of the top-quark Yukawa coupling are proposed. These observables are defined in terms of the transverse and longitudinal projections of $t$, $\bar{t}$ and $h$ momenta with respect to the beam axis in the $t{\bar t} h$ rest frame. Distributions differential with respect to those observables are evaluated up to NLO in QCD. These distributions are found to be sensitive to the $CP$ nature of the coupling. Dileptonic final states of the $t{\bar t} h$ system (with $h\rightarrow b\bar b$) are used, after fast DELPHES detector simulation and full event reconstruction through a kinematic fit, as a case study to test the observables' sensitivity to the $CP$ nature of the coupling. Confidence levels are presented as a function of the total integrated LHC luminosity for the case of exclusion of a pure $CP$-odd coupling against the Standard Model $CP$-even hypothesis. By using observables evaluated in the $t{\bar t} h$ system, the luminosity needed to directly probe the $CP$ properties of the top-quark Yukawa coupling at the High-Luminosity run of the LHC can be decreased by a few hundred inverse femtobarns, when compared to analyses that use observables in the laboratory rest frame. In addition, transverse momentum distributions of the $h$ boson and top quarks are found to provide no more discriminant power than a counting experiment.

hep-ph↗
arXiv2019

Measuring the CP structure of the top Yukawa coupling in $t\bar{t}H$ events at the LHC

The ATLAS and CMS collaborations recently announced the observation of the associated production of the Higgs boson with a top quark pair ($t\bar tH$) at the LHC. This process depends directly on the the top quark Yukawa coupling and provides access to its properties. In particular, a CP-odd component is allowed in models beyond the Standard Model with extended Higgs sectors. Studies of the feasibility of such a measurement at the 13 TeV LHC were carried out, in the $H\rightarrow b\bar b$ decay channel and with the $t\bar t$ system decaying through the semileptonic and dileptonic channels. Fast detector simulation and kinematic fits were applied to samples of SM backgrounds and of signal scenarios with different CP-mixing angles of the coupling. Ratios of projections of momenta and angular distributions using boosted reference frames were found to be sensitive to the CP-mixing angle. Those are expected to be robust relatively to modeling uncertainties and are thus presented as good candidates for experimental use. Expected confidence levels for the exclusion of scenarios with a CP-odd component in the top quark Yukawa coupling were obtained, using different observables as discriminants, and are presented for integrated luminosities up to 3 ab$^{-1}$, as expected after the full HL-LHC program.

hep-ph↗
arXiv2019

Reconstruction of top quark pair dilepton decays in electron-positron collisions

A new algorithm is presented to perform the full kinematic reconstruction of top quark pair events produced at future electron-positron colliders in the case of dilepton decays of the $W$ bosons to electrons or muons. The momentum components of the undetected neutrino and anti-neutrino in the event are reconstructed by employing several kinematic conditions comprising a non-linear system of six equations. This system is solved numerically using two independent methods and the selection of the best candidate real solution for each event is determined by a likelihood discriminant. Results are presented for several reconstructed kinematic properties of the $W^\pm$ bosons, top (and anti-top) quarks using generator level information produced at leading order.

hep-ex↗
arXiv2018

Top Quark Anomalous Couplings at the High-Luminosity Phase of the LHC

The combination of the latest and most precise measurements of several top quark properties is presented in this paper in order to establish allowed regions on anomalous contributions to the Lorentz structure of the $Wtb$ vertex. These measurements include single top production cross sections, $W$ boson helicity fractions and forward-backward asymmetries, both at Tevatron and at the Large Hadron Collider, up to a center-of-mass energy of 13 TeV. The results obtained at 95% Confidence Level for the top quark anomalous couplings are compared with the limits extracted from a combination that includes the expected measurements at the future High-Luminosity run of the Large Hadron Collider.

hep-ph↗