arxiv: 2605.02341 · v1 · submitted 2026-05-04 · ✦ hep-ex

Recognition: unknown

Study of tbar{t} threshold effects in eμ differential distributions measured in sqrt{s}=13\,TeV pp collisions with the ATLAS detector

ATLAS Collaboration

Authors on Pith no claims yet

Pith reviewed 2026-05-08 02:46 UTC · model grok-4.3

classification ✦ hep-ex

keywords top quarkttbar thresholdquasi-bound statesdilepton distributionsATLASLHC13 TeV

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The pith

The ATLAS eμ data at 13 TeV are better described by ttbar models that include color-singlet quasi-bound states near threshold than by models without them.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper extends an earlier ATLAS measurement of normalized differential distributions in electron-muon pairs produced in top-antitop events. It tests whether adding the formation of color-singlet quasi-bound states just below the ttbar production threshold improves agreement with the observed dilepton invariant-mass and azimuthal-angle spectra. The distributions match the augmented predictions more closely. A fit to the invariant-mass spectrum finds evidence for the quasi-bound-state contribution with a significance above three standard deviations, and the extracted cross-section is consistent with independent threshold-region studies.

Core claim

The inclusion of color-singlet quasi-bound-state formation in the perturbative QCD modeling of ttbar production improves the description of the measured normalized eμ dilepton invariant-mass and azimuthal-angle distributions in 13 TeV proton-proton collisions; fits to the invariant-mass distribution yield an observed significance exceeding three standard deviations, with the measured cross-section agreeing with dedicated threshold analyses.

What carries the argument

Color-singlet quasi-bound states near the ttbar threshold, added to perturbative QCD hard-process predictions to modify the differential distributions.

If this is right

The measured differential distributions agree better with predictions that incorporate quasi-bound-state formation.
The dilepton invariant-mass fit shows evidence exceeding three standard deviations.
The extracted cross-section for the quasi-bound-state process matches results from dedicated threshold studies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Threshold effects of this type may need to be included in future precision measurements of top-pair production to reduce modeling uncertainty.
Higher-statistics data could allow extraction of the binding energy or width of the quasi-bound state.
Similar near-threshold enhancements could appear in other heavy-quark pair processes at hadron colliders.

Load-bearing premise

That the improved description of the data arises specifically from the quasi-bound states rather than from other modeling uncertainties or background choices.

What would settle it

A new dataset or refined calculation in which adding the quasi-bound-state term no longer improves the fit quality to the dilepton mass distribution.

read the original abstract

The recent ATLAS measurement of the normalised $e\mu$ dilepton invariant mass and azimuthal angle distributions in $\sqrt{s}=13$\,TeV $pp$ collisions at the Large Hadron Collider is extended to study the sensitivity to the formation of quasi-bound states near the $t\bar{t}$ threshold. The measured differential distributions are compared with $t\bar{t}$ models incorporating perturbative QCD predictions for the hard process, with or without the addition of colour-singlet quasi-bound states. The data are better described by the predictions incorporating quasi-bound-state formation. Fits to the dilepton invariant mass distribution show evidence for the latter process with an observed significance exceeding three standard deviations, and a measured cross-section in agreement with dedicated studies of the threshold region.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

ATLAS finds >3σ preference for quasi-bound states in ttbar by fitting dilepton mass, but the improvement may not be unique to that addition.

read the letter

This ATLAS paper finds that their eμ dilepton data are better described when color-singlet quasi-bound states are added to the ttbar threshold modeling, with a fit significance over three standard deviations. They build on a recent measurement by adding this specific comparison and new fits to the invariant mass distribution. The cross-section they extract agrees with other dedicated threshold work. That part is straightforward and uses the data they already have. The main concern is whether the better description is uniquely due to the quasi-bound states. The improvement could come from the extra freedom in the model rather than a genuine requirement from the data, especially if pQCD scales, shower matching, or backgrounds weren't varied exhaustively in the baseline. The abstract leaves that open, so the full paper needs to demonstrate that the effect isn't degenerate with those choices. This is for readers focused on precision top measurements and near-threshold QCD at hadron colliders. It gives them a data-driven handle on the modeling, but the impact stays within that area. It should go through peer review. The experimental result is concrete enough that a referee can evaluate the fit procedure and uncertainty treatment properly.

Referee Report

1 major / 1 minor

Summary. The manuscript extends a prior ATLAS measurement of normalized differential distributions in the eμ final state from ttbar production in 13 TeV pp collisions to examine sensitivity to color-singlet quasi-bound state formation near the ttbar threshold. Data are compared to pQCD hard-process predictions with and without the addition of quasi-bound states; the distributions are better described when the quasi-bound-state component is included. A fit to the dilepton invariant mass distribution yields an observed significance exceeding 3 standard deviations for the quasi-bound-state process, with the extracted cross section consistent with dedicated threshold-region studies.

Significance. If the central claim holds after addressing modeling uncertainties, the result would provide experimental evidence for non-perturbative quasi-bound-state effects in top-pair production, which could refine theoretical predictions and Monte Carlo modeling near threshold. This has potential implications for precision ttbar cross-section and property measurements at the LHC. The reported consistency with independent threshold studies is a positive aspect of the analysis.

major comments (1)

[Results section (fit to dilepton invariant mass distribution)] The abstract and results section on the fit to the dilepton invariant mass distribution: the >3σ significance is extracted from a data-to-model comparison that adds the quasi-bound-state term to a baseline pQCD prediction. The manuscript does not demonstrate that this improvement is isolated to the quasi-bound-state component rather than being degenerate with variations in renormalization/factorization scales, parton-shower matching, or background normalizations in the threshold region. Explicit tests (e.g., profiling these parameters in both models and showing the resulting significance) are needed to substantiate that the data genuinely require the new mechanism.

minor comments (1)

[Abstract] The abstract refers to 'normalised eμ differential distributions' without specifying the exact normalization procedure or binning choices; this should be clarified with a reference to the relevant methods section or supplementary material for reproducibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive summary of our work and for the constructive major comment, which helps strengthen the robustness of our claims. We respond to the point below.

read point-by-point responses

Referee: The abstract and results section on the fit to dilepton invariant mass distribution: the >3σ significance is extracted from a data-to-model comparison that adds the quasi-bound-state term to a baseline pQCD prediction. The manuscript does not demonstrate that this improvement is isolated to the quasi-bound-state component rather than being degenerate with variations in renormalization/factorization scales, parton-shower matching, or background normalizations in the threshold region. Explicit tests (e.g., profiling these parameters in both models and showing the resulting significance) are needed to substantiate that the data genuinely require the new mechanism.

Authors: We agree that explicit demonstration of non-degeneracy is required to substantiate the claim that the data prefer the quasi-bound-state mechanism. The baseline pQCD model in the manuscript uses standard central-scale choices (μ_R = μ_F = m_t) and parton-shower settings, with scale and matching variations already evaluated as systematic uncertainties and propagated into the fit. The quasi-bound-state addition provides a specific shape distortion localized to the threshold region that is not reproduced by uniform scale shifts or background rescaling. Nevertheless, we acknowledge that the current manuscript does not include the requested profiling exercise. We will therefore add these tests in the revised version: both models will be refitted with the listed parameters profiled, and the resulting significance and likelihood contours will be shown to confirm that the >3σ preference for the quasi-bound-state component persists. revision: yes

Circularity Check

0 steps flagged

No circularity: empirical data-model comparison with external pQCD inputs

full rationale

The paper reports an experimental measurement of differential distributions and performs statistical fits to test whether adding a color-singlet quasi-bound-state component to standard perturbative QCD predictions improves agreement with data. The claimed evidence (>3σ significance and cross-section agreement) is obtained directly from the data-to-model comparison and likelihood fit; it does not reduce by any equation or self-citation to a quantity defined in terms of the fitted parameter itself. No self-definitional loops, fitted inputs renamed as predictions, or load-bearing self-citations appear in the abstract or described analysis chain. The baseline pQCD models and the quasi-bound-state addition are treated as external theoretical inputs whose relative descriptive power is tested against independent data. This constitutes a standard, non-circular hypothesis test.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract provides no explicit free parameters, axioms, or invented entities beyond referencing existing perturbative QCD predictions and optional quasi-bound-state models; no independent evidence for the states is supplied here.

pith-pipeline@v0.9.0 · 5434 in / 1096 out tokens · 81840 ms · 2026-05-08T02:46:33.787356+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

30 extracted references · 27 canonical work pages · 5 internal anchors

[1]

CMSCollaboration,Observationofapseudoscalarexcessatthetopquarkpairproductionthreshold, Rept. Prog. Phys.88(2025) 087801, arXiv:2503.22382 [hep-ex]

work page arXiv 2025
[2]

ATLASCollaboration,Observationofacross-sectionenhancementnearthe 𝑡¯𝑡 productionthreshold in √𝑠=13TeV𝑝 𝑝collisions with the ATLAS detector, (2026), arXiv:2601.11780 [hep-ex]

work page internal anchor Pith review arXiv 2026
[3]

ATLASCollaboration,Precisemeasurementofthe 𝑡¯𝑡productioncross-sectionandleptondifferential distributions in𝑒𝜇 dilepton events from√𝑠=13TeV𝑝 𝑝 collisions with the ATLAS detector, (2025), arXiv:2509.15066 [hep-ex]

work page arXiv 2025
[4]

ATLAS Collaboration,Measurement of the 𝑡¯𝑡 production cross-section and lepton differential distributions in𝑒𝜇 dilepton events from𝑝 𝑝collisions at√𝑠=13TeV with the ATLAS detector, Eur. Phys. J. C80(2020) 528, arXiv:1910.08819 [hep-ex]

work page arXiv 2020
[5]

ATLAS Collaboration,Measurement of lepton differential distributions and the top quark mass in𝑡¯𝑡 production in𝑝 𝑝collisions at √𝑠=8TeV with the ATLAS detector, Eur. Phys. J. C77(2017) 804, arXiv:1709.09407 [hep-ex]

work page arXiv 2017
[6]

CMS Collaboration,Measurements of 𝑡¯𝑡 differential cross sections in proton–proton collisions at √𝑠=13TeV using events containing two leptons, JHEP02(2019) 149, arXiv:1811.06625 [hep-ex]

work page arXiv 2019
[7]

B. Fuks, A. Hossain and J. Keaveney,Statistical Indications of Toponium Formation in Top Quark Pair Production, Phys. Lett. B873(2026) 140179, arXiv:2511.02040 [hep-ph]

work page arXiv 2026
[8]

HEPData high-energy physics data repository,url: https://www.hepdata.net/record/ ins1759875
[9]

ATLAS Collaboration,The ATLAS Experiment at the CERN Large Hadron Collider, JINST3 (2008) S08003

2008
[10]

Abbott et al.,Production and integration of the ATLAS Insertable B-Layer, JINST13(2018) T05008, arXiv:1803.00844 [physics.ins-det]

B. Abbott et al.,Production and integration of the ATLAS Insertable B-Layer, JINST13(2018) T05008, arXiv:1803.00844 [physics.ins-det]

work page arXiv 2018
[11]

ATLAS Collaboration,ATLAS Insertable B-Layer: Technical Design Report, ATLAS-TDR-19; CERN-LHCC-2010-013, 2010,url: https://cds.cern.ch/record/1291633 , Addendum: ATLAS-TDR-19-ADD-1; CERN-LHCC-2012-009, 2012,url:https://cds.cern.ch/record/ 1451888

work page arXiv 2010
[12]

ATLAS Collaboration,The ATLAS Collaboration Software and Firmware, ATL-SOFT-PUB-2021- 001, 2021,url:https://cds.cern.ch/record/2767187

work page arXiv 2021
[13]

A Positive-Weight Next-to-Leading-Order Monte Carlo for Heavy Flavour Hadroproduction

S. Frixione, G. Ridolfi and P. Nason,A Positive-weight next-to-leading-order Monte Carlo for heavy flavour hadroproduction, JHEP09(2007) 126, arXiv:0707.3088 [hep-ph]

work page Pith review arXiv 2007
[14]

An Introduction to PYTHIA 8.2

T. Sjöstrand et al.,An introduction to PYTHIA 8.2, Comput. Phys. Commun.191(2015) 159, arXiv: 1410.3012 [hep-ph]

work page internal anchor Pith review arXiv 2015
[15]

Top-pair production at the LHC through NNLO QCD and NLO EW

M. Czakon et al.,Top-pair production at the LHC through NNLO QCD and NLO EW, JHEP10 (2017) 186, arXiv:1705.04105 [hep-ph]

work page Pith review arXiv 2017
[16]

ATLAS Collaboration,Measurements of differential cross-sections of𝑊 𝑏𝑊 𝑏production in the dilepton channel in 𝑝 𝑝 collisions at √𝑠=13TeV using the ATLAS detector, (2025), arXiv: 2506.14700 [hep-ex]. 13

work page arXiv 2025
[17]

J.Mazzitellietal.,Next-to-Next-to-LeadingOrderEventGenerationforTop-QuarkPairProduction, Phys. Rev. Lett.127(2021) 062001, arXiv:2012.14267 [hep-ph]

work page arXiv 2021
[18]

A New Method for Combining NLO QCD with Shower Monte Carlo Algorithms

P. Nason,A new method for combining NLO QCD with shower Monte Carlo algorithms, JHEP11 (2004) 040, arXiv:hep-ph/0409146

work page internal anchor Pith review arXiv 2004
[19]

Matching NLO QCD computations with Parton Shower simulations: the POWHEG method

S. Frixione, P. Nason and C. Oleari,Matching NLO QCD computations with parton shower simulations: the POWHEG method, JHEP11(2007) 070, arXiv:0709.2092 [hep-ph]

work page internal anchor Pith review arXiv 2007
[20]

A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX

S. Alioli, P. Nason, C. Oleari and E. Re,A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX, JHEP06(2010) 043, arXiv:1002.2581 [hep-ph]

work page internal anchor Pith review arXiv 2010
[21]

T. Ježo, J. M. Lindert, P. Nason, C. Oleari and S. Pozzorini,An NLO+PS generator for𝑡¯𝑡 and 𝑊𝑡 productionanddecayincludingnon-resonantandinterferenceeffects, Eur.Phys.J.C76(2016)691, arXiv:1607.04538 [hep-ph]

work page arXiv 2016
[22]

NNPDF Collaboration,Parton distributions for the LHC Run II, JHEP04(2015) 040, arXiv: 1410.8849 [hep-ph]

work page Pith review arXiv 2015
[23]

ATLAS Collaboration,ATLAS Pythia 8 tunes to7TeV data, ATL-PHYS-PUB-2014-021, 2014, url:https://cds.cern.ch/record/1966419

work page arXiv 2014
[24]

D.J.Lange,TheEvtGenparticledecaysimulationpackage, Nucl.Instrum.Meth.A462(2001)152

2001
[25]

B. Fuks, K. Hagiwara, K. Ma and Y.-J. Zheng,Simulating toponium formation signals at the LHC, Eur. Phys. J. C85(2025) 157, arXiv:2411.18962 [hep-ph]

work page arXiv 2025
[26]

MadGraph 5 : Going Beyond

J. Alwall, M. Herquet, F. Maltoni, O. Mattelaer and T. Stelzer,MadGraph 5 : going beyond, JHEP 06(2011) 128, arXiv:1106.0522 [hep-ph]

work page Pith review arXiv 2011
[27]

B. Fuks, K. Hagiwara, K. Ma and Y.-J. Zheng,Signatures of toponium formation in LHC run 2 data, Phys. Rev. D104(2021) 034023, arXiv:2102.11281 [hep-ph]

work page arXiv 2021
[28]

Herwig++ Physics and Manual

M. Bähr et al.,Herwig++ physics and manual, Eur. Phys. J. C58(2008) 639, arXiv:0803.0883 [hep-ph]

work page Pith review arXiv 2008
[29]

Herwig 7.1 Release Note

J. Bellm et al.,Herwig 7.1 Release Note, (2017), arXiv:1705.06919 [hep-ph]

work page Pith review arXiv 2017
[30]

Demokritos

ATLAS Collaboration,ATLAS Computing Acknowledgements, ATL-SOFT-PUB-2026-001, 2026, url:https://cds.cern.ch/record/2952666. 14 The ATLAS Collaboration G. Aad 102, E. Aakvaag 17, B. Abbott 121, S. Abdelhameed 83b, K. Abeling 54, N.J. Abicht 48, S.H. Abidi 30, M. Aboelela 44, A. Aboulhorma 36e, H. Abramowicz 154, B.S. Acharya 68a,68b,m, A.Ackermann 62a, J.Ac...

work page arXiv 2026