arXiv subjects

Hesham El Faham

Publications and source records attributed to Hesham El Faham.

10 recordsLinked to original sources

arXiv2026

The inseparable three and four tops

In measurements of four-top-quark production ($tttt$), LHC collaborations observe a significant degeneracy with three-top-quark production. We compute the dominant three-top-production mode, namely associated production with a $W$ boson ($tttW$), at complete next-to-leading order (NLO), including all possible QCD and electroweak (EW) corrections. Beyond leading order (LO), $tttW$ production with the radiation of an additional $b$-flavoured quark contributes to the same final state as $tttt$ production with a $t \to bW$ decay. Away from the on-shell top-quark limit, the usual overlap removal of resonant contributions in the non-resonant computation either breaks gauge invariance and generates unitarity violation or involves a significant arbitrariness in the required reshuffling of momenta. To overcome these issues, we introduce a novel window-removal prescription that produces consistent predictions for the inseparable $tttW+tttt$ process, with both components described at NLO accuracy. We argue that such a joint prediction should be used in comparisons with experimental selections targeting $tttt$ production, since the on-shell $tttt$ component can not be isolated in practice. Such a joint prediction has an inclusive rate more than 10% higher than the purely on-shell $tttt$ one. We also study an idealised veto on additional hard and central $b$-jet radiation, which suppresses the contributions of resonant $tttt$ diagrams as well as their interference with non-resonant $tttW$ ones and therefore defines a relatively pure $tttW$-like signal region. Formally subleading coupling orders are numerically important at LO, while the corresponding subleading NLO corrections largely cancel both inclusively and differentially. Consequently, the complete-NLO prediction is well approximated by retaining the first three LO coupling orders together with the leading QCD NLO correction.

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arXiv2026

Constraining four-heavy-quark operators with top-quark, Higgs, and electroweak precision data

We establish constraints on the dimension-six four-heavy-quark operators in the Standard Model Effective Field Theory (SMEFT) by synthesising LHC measurements of top-quark and single-Higgs production with electroweak precision observables. We scrutinise the choice of the $γ_5$ scheme in single-Higgs calculations, demonstrating its non-negligible impact on SMEFT fits.

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arXiv2026

Triple-gauge couplings in LHC diboson production: a SMEFT view from every angle

This study explores fully leptonic WZ and WW production at the LHC within the SMEFT framework at NLO in QCD, focusing on both CP-even and CP-odd triple-gauge-coupling dimension-six operators. We investigate the off-shell processes, contrasting our findings in inclusive setups with those in the presence of realistic fiducial selections. Alongside the conventional kinematic observables, we examine polarisation-sensitive observables and angular coefficients. Moreover, we assess potential SMEFT effects on asymmetry observables. Through a sensitivity analysis, we identify critical LHC observables that are particularly sensitive to SMEFT-induced modifications, thereby shedding light on potential avenues for new-physics searches in diboson production at the LHC.

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arXiv2025

Electroweak corrections in the SMEFT: four-fermion operators at high energies

In the Standard Model (SM), electroweak (EW) corrections become significant at high energies, particularly at the tera-electronvolt scale and beyond, due to the presence of Sudakov logarithms. At these energy scales, the Standard Model Effective Field Theory (SMEFT) framework provides an enhanced sensitivity to potential new physics effects. This motivates the inclusion of EW corrections not only for SM predictions but also for analyses within SMEFT. In this work, we compute EW corrections in the high-energy limit for a selected set of dimension-six operators, specifically the class of four-fermion contact interactions, in key hard-scattering processes relevant to both current and future colliders: top-quark pair production at the Large Hadron Collider (LHC) and in a muon collider scenario, as well as the Drell-Yan process at the LHC. We first discuss the technical details and challenges associated with evaluating EW Sudakov logarithms in SMEFT, contrasting them with the SM case. We then present phenomenological results for the aforementioned processes, highlighting the non-trivial effects introduced by EW corrections arising from the insertion of dimension-six, four-fermion operators. Importantly, the resulting $K$-factors exhibit significant deviations from their SM counterparts, with dependencies not only on the process but also on the specific operators considered. Finally, we explore the potential to lift flat directions in the SMEFT parameter space by incorporating higher-order corrections, using Fisher information techniques.

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arXiv2025

Diboson production in the SMEFT at dimension-8

We present a comprehensive analysis of dimension-8 and dimension-6 effects in fully leptonic $WZ$ and $WW$ production at the Large Hadron Collider (LHC) within the Standard Model (SM) Effective Field Theory (SMEFT). We focus on dimension-8 operators with maximal energy growth in the quark-(anti)quark-initiated production channel and assess their impact differentially through a variety of observables, including polarisation-sensitive ones. Leveraging existing data from measurements at the LHC, we perform fits to quantify the sensitivity of current and future data to dimension-8 effects and evaluate their interplay with squared dimension-6 contributions. By marginalising over the dimension-8 operators we examine the robustness of a dimension-6 SMEFT analysis in diboson production. We find that dimension-8 effects become subdominant only for new-physics scales above 3 TeV.

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arXiv2023

Exploiting exotic LHC datasets for long-lived new particle searches

Motivated by the expectation that new physics may manifest itself in the form of very heavy new particles, most of the operation time of the LHC is devoted to $pp$ collisions at the highest achievable energies and collision rates. The large collision rates imply tight trigger requirements that include high thresholds on the final-state particles' transverse momenta $p_{T}$ and an intrinsic background in the form of particle pileup produced by different collisions occurring during the same bunch crossing. This strategy is potentially sub-optimal for several well-motivated new physics models where new particles are not particularly heavy and can escape the online selection criteria of the multi-purpose LHC experiments due to their light mass and small coupling. A solution may be offered by complementary datasets that are routinely collected by the LHC experiments. These include heavy ion collisions, low-pileup runs for precision physics, and the so-called 'parking' and 'scouting' datasets. While some of them are motivated by other physics goals, they all have the usage of mild $p_{T}$ thresholds at the trigger-level in common. In this study, we assess the relative merits of these datasets for a representative model whose particular clean signature features long-lived resonances yielding displaced dimuon vertices. We compare the reach across those datasets for a simple analysis, simulating LHC data in Run 2 and Run 3 conditions with the Delphes simulation. We show that the scouting and parking datasets, which afford low-$p_{T}$ trigger thresholds by only using partial detector information and delaying the event reconstruction, respectively, have a reach comparable to the standard $pp$ dataset with conventional thresholds. We also show that heavy ion and low-pileup datasets are far less competitive for this signature.

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arXiv2022

Complete study of four top quark production at the LHC in the SMEFT

We present a study of four top quark production at the LHC in the Standard Model Effective Field Theory (SMEFT). The analysis is performed at the tree-level, including all terms from the mixed QCD-EW-coupling cross-section expansion and all the relevant dimension-six SMEFT operators. We find several cases in which formally subleading terms give rise to significant contributions. Moreover, we obtain limits on the SMEFT Wilson coefficients (WCs), including different collider setups, FCC-hh and HL-LHC, using a simplified chi-square fit. Through the latter, we also assessed the importance of including differential information in the fit.

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arXiv2022

Complete SMEFT predictions for four top quark production at hadron colliders

We study four top quark production at hadron colliders in the Standard Model Effective Field Theory (SMEFT). We perform an analysis at the tree-level, including all possible QCD- and EW-coupling orders and relevant dimension-six operators. We find several cases where formally subleading terms give rise to significant contributions, potentially providing sensitivity to a broad class of operators. Inclusive and differential predictions are presented for the LHC and a future $pp$ circular collider operating at $100$ TeV. We estimate the sensitivity of different operators and perform a simplified chi-square fit to set limits on SMEFT Wilson coefficients. In so doing, we assess the importance of including subleading terms and differential information in constraining new physics contributions. Finally, we compute the SMEFT predictions for the double insertion of dimension-six operators and scrutinise the possible enhancements to the sensitivity induced by a specific class of higher order terms in the EFT series.

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arXiv2022

Single top production in association with a $WZ$ pair at the LHC in the SMEFT

We study single top quark production in association with a $WZ$ pair at the LHC in the context of the Standard Model (SM) and the Standard Model Effective Field Theory (SMEFT). A significant advantage of $tWZ$ compared to other EW top production processes is its sensitivity to unitarity-violating behaviour induced in its $2\to2$ sub-amplitudes through modified EW interactions. At NLO in QCD, $tWZ$ interferes with $t\bar{t}Z$ and $t\bar{t}$ and a method to meaningfully separate it from these overlapping processes needs to be employed. In order to define $tWZ$ production for total rates and differential distributions, we consider the approaches proposed in the literature for similar cases and find that diagram-removal procedures provide reliable results both for the SM and the SMEFT in a suitably defined phase-space region. We provide robust results for total and differential cross sections for $tWZ$ at 13 TeV, including the six relevant dimension-6 operators ($\mathcal{O}_{tW}$,$\mathcal{O}_{tZ}$,$\mathcal{O}_{tG}$,$\mathcal{O}_{ϕQ}^{(-)}$,$\mathcal{O}_{ϕQ}^{(3)}$,$\mathcal{O}_{ϕt}$), also matching short-distance events to parton shower.

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arXiv2021

$tWZ$ production at NLO in QCD in the SMEFT

At NLO in QCD, the $tWZ$ production process interferes with resonant LO processes, $t\bar{t}Z$ and $t\bar{t}$, and a method to meaningfully disentangle these overlapping processes needs to be employed. I discuss the diagram-removal procedure needed to obtain an operative definition of $tWZ$ at NLO accuracy. I also show the predictions we obtain in the context of the Standard Model and in the Standard Model Effective Field Theory including the relevant dimension-six SMEFT operators.

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