Publications

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Top observables as precise probes of the ALP

Anh Vu Phan

SciPost Phys. Proc. 18, 022 (2026) · published 29 January 2026 | · pdf

Measurements of the top quark by the ATLAS and CMS experiments go beyond testing the Standard Model (SM) with high precision. Axion-like particles (ALPs), a potential SM extension involving new pseudoscalar particles, exhibit strong interactions with heavy SM fermions. Consequently, they can significantly affect the kinematic distributions of top quarks in top-antitop pair production. Moreover, such strong interactions can induce other ALP couplings at low energies, leading to a rich phenomenology. We summarize recent developments in probing the ALP-top coupling and use LHC data from run 2 to constrain the ALP parameter space.
Proposal for simplified template cross-sections extension using $\mathcal{CP}$ observables in $t\overline{t}H$

Carnelli Alberto

SciPost Phys. Proc. 18, 021 (2026) · published 29 January 2026 | · pdf

The Large Hadron Collider (LHC) offers a unique opportunity to investigate $\mathcal{CP}$ violation in the Yukawa coupling between the Higgs boson and the top quark by studying Higgs production in association with top quarks; this is of fundamental importance, seeing that the $\mathcal{CP}$ properties of the Higgs boson are yet to measure with high precision. To address this, the focus of this work has been an extension of the simplified template cross-section (STXS) framework, devised to be sensitive to $\mathcal{CP}$ effects. Our study focused on $\mathcal{CP}$-sensitive observables across multiple Higgs decay channels, comparing their performances. The result indicates that the most efficient extension of the current binning used in the STXS framework, which currently uses the Higgs boson's transverse momentum $p_{T,H}$, requires adding one further split using $\mathcal{CP}$-sensitive observables. Between these observables, one of the best is the Collins-Soper angle $|\cos\theta^*|$, a variable derived from momenta information of the top quarks. We have investigated the improvement brought by our two-dimensional STXS setup and compared it to the currently employed methodologies, finding an increase in performances at an integrated luminosity of $300 \mathrm{ fb}^{-1}$. Moreover, our results highlight that this advantage seems to be present also at $3000 \mathrm{ fb}^{-1}$.
Search for heavy scalar or pseudoscalar states in $\mathrm{t \bar{t}}$ events at CMS

Laurids Jeppe, on behalf of the CMS collaboration

SciPost Phys. Proc. 18, 020 (2026) · published 29 January 2026 | · pdf

A search for scalar or pseudoscalar states decaying to a top quark-antiquark pair ($\mathrm{t \bar{t}}$), using $138 \mathrm{ fb}^{-1}$ of pp collision data taken at $\sqrt{s} = 13 \mathrm{ TeV}$ using the CMS detector, is presented. Events with one or two leptons are analyzed using the invariant $\mathrm{t \bar{t}}$ mass ($m_{\mathrm{t \bar{t}}}$) as well as angular and spin correlation observables. An excess in the data is observed for low values of $m_{\mathrm{t \bar{t}}}$, preferring a pseudoscalar over a scalar hypothesis. It is interpreted in terms of a generic model of (pseudo)scalar boson production, as well as a simplified model of a $\mathrm{t \bar{t}}$ bound state ($\eta_{\mathrm{t}}$), yielding good agreement with the data. Moreover, limits on the couplings of additional (pseudo)scalar bosons to top quarks are set.
Search for flavour-changing neutral current couplings between the top quark and the Higgs boson in multilepton final states with the ATLAS detector

Shayma Wahdan, on behalf of the ATLAS collaboration

SciPost Phys. Proc. 18, 019 (2026) · published 29 January 2026 | · pdf

These proceedings present a search for flavour-changing neutral-current (FCNC) interaction involving the top quark, Higgs boson and either the up or the charm quark, using 140 $\text{fb}^{-1}$ of 13 TeV proton--proton collision data from the ATLAS detector at the Large Hadron Collider. Two channels are considered: the production of top quark-antiquark pair with one top decaying via FCNC, and the associated production of a top quark and Higgs boson. Final states contain either two same-charge leptons, or three leptons of which two have the same charge. Observed (expected) upper limits on the branching rations are determined as $\mathcal{B}(t\to Hu)<2.8 (3.0) × 10^{-4}$ and $\mathcal{B}(t\to Hc)<3.3 (3.8) × 10^{-4}$.
Probes of flavour symmetry and violation with top quarks in ATLAS and CMS

Miriam Watson, on behalf of the ATLAS and CMS collaborations

SciPost Phys. Proc. 18, 018 (2026) · published 29 January 2026 | · pdf

Results are presented of searches and measurements in the top quark sector by the ATLAS and CMS experiments. These analyses use data from proton-proton collisions at a centre-of-mass energy of $13$ TeV, recorded during Run 2 at the Large Hadron Collider and corresponding to integrated luminosities of 138-140 fb$^{-1}$. Searches are carried out for charged lepton flavour violation, baryon number violation and the presence of neutral heavy leptons. A precise measurement of lepton flavour universality between electrons and muons originating from top quark-antiquark events is also presented.
The magic of top quarks

Chris D. White, Martin J. White

SciPost Phys. Proc. 18, 017 (2026) · published 29 January 2026 | · pdf

In recent years, there has been increasing collaboration between the fields of quantum computing and high energy physics, including using LHC processes such as top (anti-)quark pair production to perform high energy tests of quantum entanglement. In this proceeding, I will review another interesting property from quantum computing ("magic"), that is needed to make quantum computers with genuine computational advantage over their classical counterparts. How to make and enhance magic in general quantum systems is an open question, such that new insights are always useful. To this end, we will show that the LHC naturally produces magic top quarks, providing a novel playground for further study in this area.
Top quark spin and quantum entanglement in the ATLAS experiment

Roman Lysak, on behalf of the ATLAS collaboration

SciPost Phys. Proc. 18, 016 (2026) · published 29 January 2026 | · pdf

Top quark pair spin correlation measurements performed by the ATLAS experiment using $pp$ collisions at the CERN Large Hadron Collider are summarized. Moreover, the measurement of a specific observable $D$ related to top quark pair spin correlations is presented using the full LHC Run 2 data taking at the center-of-mass energy of $\sqrt{s} = 13$~TeV. This allowed the ATLAS experiment to observe the quantum entanglement, one of the fundamental property of the quantum mechanics.
$t\bar{t}t\bar{t}$: NLO QCD corrections in production and decays for the $3\ell$ channel

Nikolaos Dimitrakopoulos

SciPost Phys. Proc. 18, 015 (2026) · published 29 January 2026 | · pdf

We discuss the results for the four-top quark production process at the LHC at NLO accuracy in perturbative QCD for the $3\ell$ decay channel. The QCD corrections are applied in both the production and the decay stages of the four top quarks by employing the narrow-width approximation. The spin correlations are therefore preserved at NLO accuracy in QCD without any approximation. We summarize the impact of higher-order QCD effects by highlighting the sensitivity of the results on the cut applied on the invariant mass of the two hardest light jets in the process.
Predictions for off-shell $t\bar{t}Z$ production at the LHC: The complete set of LO and NLO contributions

Daniele Lombardi

SciPost Phys. Proc. 18, 014 (2026) · published 29 January 2026 | · pdf

We present predictions for the production and decay of a top–antitop pair in association with a $\rm Z$ boson in the multi-lepton decay channel at the LHC. Our results include the complete set of LO and NLO contributions. Since our calculation is based on full matrix elements, off-shell effects are entirely taken into account. Integrated and differential cross-sections are reported for a realistic fiducial setup.
Complete NLO corrections to $t\bar{t}\gamma$ and $t\bar{t}\gamma\gamma$

Daniel Stremmer

SciPost Phys. Proc. 18, 013 (2026) · published 29 January 2026 | · pdf

In this contribution we discuss recent progress in associated top-quark pair production with one or two isolated photons, $pp\to t\bar{t}\gamma(\gamma)$. The focus is the simultaneous inclusion of higher-order effects and photon radiation in the production of the top-quark pair and in the decay processes. This allows us to quantify the importance of photon radiation in decay processes and the size of the so-called complete NLO corrections in realistic final states.

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