Publications

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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.
Towards a differential $\mathrm{t\bar{t}W}$ cross section measurement at CMS

David Marckx, on behalf of the CMS collaboration

SciPost Phys. Proc. 18, 009 (2026) · published 29 January 2026 | · pdf

Top quark pair production in association with a W boson is a rare standard model process that has proven to be an intriguing puzzle for theorists and experimentalists alike. Recent measurements, performed at $\sqrt{s}$ = 13 TeV, by both the ATLAS and CMS Collaborations at the CERN LHC, find cross section values that are consistently higher than the latest state-of-the-art theory predictions. In this presentation, both experimental and theoretical challenges in the pursuit of a better understanding of this process are discussed. Furthermore, a framework for a future differential measurement to be performed with the Run 2 CMS data (collected in 2016-2018) is proposed.
Inclusive top cross sections in ATLAS

Charlie Chen, on behalf of the ATLAS collaboration

SciPost Phys. Proc. 18, 008 (2026) · published 29 January 2026 | · pdf

The ATLAS collaboration at the LHC has published inclusive cross-section measurements for the single-top and $t\overline{t}$ production modes at center-of-mass energies of $\sqrt{s} = 5.02, 8.16$, $13$, and $13.6$ TeV. Single-top measurements are conducted in the $t$-channel and $tW$ channel. In addition to the nominal cross-section measurements, various measurements of other interesting observables such as the $V_{tb}$ element of the Cabibbo Kobayashi Maskawa (CKM) quark-mixing matrix, the ratio of the inclusive cross-sections between $tq$ and $t\overline{q}$, the ratio of inclusive cross-sections between $t\overline{t}$ and $Z→ \ell\ell$, and the nuclear modification factor (defined as the ratio of the inclusive $t\overline{t}$ cross section in heavy-ion collisions to the inclusive $t\overline{t}$ cross-section in $pp$ collisions) are also reported. These results are compared to their corresponding SM predictions, calculated at (N)NLO in QCD. All results are in good agreement with SM predictions.
Towards two-loop QCD corrections to $ pp \to t \bar{t} j$

Colomba Brancaccio

SciPost Phys. Proc. 18, 007 (2026) · published 29 January 2026 | · pdf

I discuss the status of the computation of the two-loop QCD corrections to top-quark pair production associated with a jet at hadron colliders. This amplitude is a missing ingredient for next-to-next-to-leading order (NNLO) QCD predictions. I briefly present computational techniques to tackle the algebraic and analytic complexities of two-loop multi-scale amplitudes, in particular where massive propagators give rise to elliptic Feynman integrals. I then describe how a special function basis for the helicity amplitudes is obtained and present first numerical evaluations for the finite remainders of the $gg\to t\bar{t}g$ channel, after the infrared and ultraviolet poles have been identified analytically.
Measurement of the $t$-channel single top-quark production cross section at $\sqrt{s}=13$ TeV with the ATLAS detector and interpretations of the measurement

Maren Stratmann, on behalf of the ATLAS collaboration

SciPost Phys. Proc. 18, 006 (2026) · published 29 January 2026 | · pdf

The $t$-channel is the dominant production channel for single top-quarks at the LHC. The total cross section of this process is measured by ATLAS in proton-proton collisions at a center-of-mass energy $\sqrt{s}=13$ TeV. The production cross sections for single top-quarks and single top-antiquarks are measured to be $\sigma_{tq}=\text{137}^{+8}_{-8}\,$pb and $\sigma_{\bar{t}q}=\text{84}^{+6}_{-5}\,$pb. For the combined cross section and the ratio of the cross sections $R_t$, $\sigma_{tq+\bar{t}q}=\text{221}^{+13}_{-13}\,$pb and $R_t=\text{1.636}^{+0\text{.}036}_{-0\text{.}034}$ are obtained. As interpretations of the measurement, limits are set on the EFT operator $O_{Qq}^{3,1}$ and on the CKM matrix elements $|V_{td}|$, $|V_{ts}|$ and $|V_{tb}|$.
Encoding off-shell effects in top pair production in direct diffusion networks

Mathias Kuschick

SciPost Phys. Proc. 18, 005 (2026) · published 29 January 2026 | · pdf

To meet the precision targets of upcoming LHC runs in the simulation of top pair production events it is essential to also consider off-shell effects. Due to their great computational cost I propose to encode them in neural networks. For that I use a combination of neural networks that take events with approximate off-shell effects and transform them into events that match those obtained with full off-shell calculations. This was shown to work reliably and efficiently at leading order. Here I discuss first steps extending this method to include higher order effects.
Measurements of $t\bar{t}$ in association with charm quarks at 13 TeV with the ATLAS experiment

Knut Zoch

SciPost Phys. Proc. 18, 004 (2026) · published 29 January 2026 | · pdf

This talk presents the ATLAS Collaboration's first measurement of the inclusive cross-section for top-quark pair production in association with charm quarks. Using the full Run~2 proton--proton collision data sample at $\sqrt{s} = 13\,\text{TeV}$, collected with the ATLAS experiment at the LHC between 2015 and 2018, the measurement selects $t\bar{t}$ events with one or two charged leptons and at least one additional jet in the final state. A custom flavour-tagging algorithm is employed to simultaneously identify $b$-jets and $c$-jets. The fiducial cross-sections for $t\bar{t}+{≥}2c$ and $t\bar{t}+1c$ production are found to largely agree with predictions from various $t\bar{t}$ simulations, though all underpredict the observed values.
Recent measurements of top cross sections at CMS

Javier del Riego Badas, on behalf of the CMS collaboration

SciPost Phys. Proc. 18, 003 (2026) · published 29 January 2026 | · pdf

This contribution presents recent cross section measurements for top quark pair production ($\mathrm{t\bar{t}}$) and single top quark production in association with a W boson (tW) in proton-proton collisions with the CMS experiment at LHC. For $\mathrm{t\bar{t}}$ production, data from 2017 at a center-of-mass (CM) energy of 5.02 TeV with an integrated luminosity of 302 pb$^{-1}$ were analyzed, with a measured inclusive cross section of $62.3 ± 1.5 \mathrm{(stat)} ± 2.4 \mathrm{(syst)} ± 1.2 \mathrm{(lumi)}$ pb, consistent with Standard Model (SM) predictions. The tW measurement was performed using data from 2022 with an integrated luminosity of 34.7 fb$^{-1}$ at a CM energy of 13.6 TeV, yielding a cross section of $82.3 ± 2.1 \mathrm{(stat)} ^{+9.9}_{-9.7} \mathrm{(syst)} ± 3.3 \mathrm{(lumi)}$ pb, in agreement with SM predictions. It includes also differential cross sections which are in good agreement with next-to-leading order in perturbative quantum chromodynamics.

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