Publications

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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.
Photon production in top quark events at ATLAS and CMS

Beatriz R. Lopes, on behalf of the ATLAS and CMS collaborations

SciPost Phys. Proc. 18, 012 (2026) · published 29 January 2026 | · pdf

Top quark production in association with a photon offers a unique test ground for the standard model predictions, as it is sensitive to the top-photon coupling. These processes are rare when compared to standard top pair production, however the large amounts of data delivered by the LHC open the window to precise measurements. This talk covered the recent inclusive and differential measurements of top quark single and pair production in association with a photon, by the ATLAS and CMS collaborations. Potential modifications to the top-photon couplings with respect to the standard model predictions are also explored using the standard model effective field theory.
Transformer neural networks in the measurement of $t\bar{t}H$ production in the $H\,{\to}\,b\bar{b}$ decay channel with ATLAS

Chris Scheulen, on behalf of the ATLAS collaboration

SciPost Phys. Proc. 18, 011 (2026) · published 29 January 2026 | · pdf

A measurement of Higgs boson production in association with a top quark pair in the bottom--anti-bottom Higgs boson decay channel and leptonic top final states is presented. The analysis uses $140\,\mathrm{fb}^{-1}$ of $13\,\mathrm{TeV}$ proton--proton collision data collected by the ATLAS detector at the Large Hadron Collider. A particular focus is placed on the role played by transformer neural networks in discriminating signal and background processes via multi-class discriminants and in reconstructing the Higgs boson transverse momentum. These powerful multi-variate analysis techniques significantly improve the analysis over a previous measurement using the same dataset. As a result, the observed (expected) event excess over the background-only hypothesis corresponds with a significance of 4.6 (5.4) standard deviations.
Measurements of inclusive and differential cross-sections of $t\bar{t}\gamma$ production in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Carmen Diez Pardos, on behalf of the ATLAS collaboration

SciPost Phys. Proc. 18, 010 (2026) · published 29 January 2026 | · pdf

Cross-section measurements of the associated production of a top quark pair and a photon ($t\bar{t}\gamma$) are performed with an integrated luminosity of 140 fb$^{-1}$ of proton−proton collisions at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. The measurement focuses on $t\bar{t}\gamma$ topologies where the photon is radiated from an initial-state parton or one of the top quarks. The differential cross-sections are measured for variables characterising the photon, lepton and jet kinematic properties. The distribution of the photon transverse momentum is used to constrain effective field theory operators related to the electroweak dipole moments of the top quark.

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