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REVIEW 1 major objections 4 minor 76 references

Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

T0 review · 1 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read This search finds no evidence for flavour-changing top-photon currents and sets the tightest limits yet on the tqγ coupling.

desk verdict A solid, incrementally new ATLAS limit on tqγ FCNC couplings; the e→γ fake-rate shape is the one caveat worth probing, but the central result holds. read the letter →

arxiv 1908.08461 v2 pith:3ZVK7TE6 submitted 2019-08-22 hep-ex

ATLAS Collaboration This is my paper · ORCID
classification hep-ex
keywords flavour-changingneutralcurrentstopquarkphotoneffectivefieldtheoryproton-protoncollisionsbranchingrationeuralnetworksingleproduction
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper searches for flavour-changing neutral currents at the coupling of a top quark, a photon, and an up or charm quark, a process that the Standard Model suppresses to branching ratios around $10^{-14}$ but that several new-physics models can enhance to observable rates. Using $81~\mathrm{fb}^{-1}$ of proton-proton collisions at 13 TeV, it selects events with a photon, a charged lepton, a $b$-tagged jet, and missing transverse momentum, and separates potential signal from background with a neural network. The data are consistent with the background-only hypothesis, so no new signal is claimed. Instead, the paper sets 95% confidence-level upper limits on the effective $tq\gamma$ coupling, on the cross section for FCNC $t\gamma$ production, and on the branching ratios $\mathcal{B}(t\to\gamma u)$ and $\mathcal{B}(t\to\gamma c)$. If correct, these are the most stringent limits to date, and they would compress the parameter space of models such as R-parity-violating supersymmetry and two-Higgs-doublet models that predict larger top-photon flavour-changing rates.

What carries the argument

The argument rests on an effective-field-theory parametrization of the $tq\gamma$ vertex through the dimension-six operators $O_{uB}$ and $O_{uW}$, whose Wilson coefficients are the parameters being constrained. Signal samples are generated at next-to-leading order in QCD with this model, and the event selection is optimized for $pp\to t\gamma$ production. A fully connected feed-forward neural network, trained on ten kinematic variables, separates signal from background, and a profile-likelihood fit to the network output in the signal region plus the $W+\gamma$ and $Z+\gamma$ control regions extracts the signal contribution. The two dominant backgrounds, electrons and hadrons misidentified as photons, are normalised with data-driven estimates: the electron fake rate $f_{e\to\gamma}=3.11\% \pm 0.01\% \pm 0.13\%$ and a hadron-fake estimate built from three hadron-fake regions via $N(\mathrm{HFR}_{\mathrm{pass}|\mathrm{fail}}) \times N(\mathrm{HFR}_{\mathrm{fail}|\mathrm{pass}}) / N(\mathrm{HFR}_{\mathrm{fail}|\mathrm{fail}})$, corrected by a simulation-derived correlation factor of $0.85\pm0.14$.

What would settle it

A tag-and-probe measurement of the electron-to-photon misidentification rate inside the signal-region phase space that disagrees with $f_{e\to\gamma}=3.11\% \pm 0.01\% \pm 0.13\%$ by more than the assigned systematic, or a future observation of a significant $pp\to t\gamma$ excess at high neural-network output, would contradict the background-only conclusion.

Watch

Extended reading notes

Core claim

The central claim is that, in the selected final state, the observed event yield agrees with the Standard Model background-only prediction, and the 95% CL upper limits derived from this agreement are the tightest published bounds on the $tq\gamma$ flavour-changing vertex. For a left-handed (right-handed) $tu\gamma$ coupling the limits are 36 fb (78 fb) on the $\sigma(pp\to t\gamma)$ production cross section and $2.8\times10^{-5}$ ($6.1\times10^{-5}$) on $\mathcal{B}(t\to\gamma u)$; for $tc\gamma$ they are 40 fb (33 fb) and $22\times10^{-5}$ ($18\times10^{-5}$). Interpreted through the dimension-six operators $O_{uB}$ and $O_{uW}$ at an effective scale $\Lambda = 1$ TeV, the limits on the coupling combinations range from 0.19 to 0.52 depending on quark flavour and handedness. The search is driven by the single-top-plus-photon production mode, but it also includes the $t\bar{t}$ decay mode, which contributes substantially to the charm-quark case because of the smaller charm-quark parton density.

Load-bearing premise

The limits assume that the probabilities for electrons and hadrons to be misidentified as photons, measured in dedicated control regions, remain valid in the signal region after the applied data-driven corrections.

Editorial extensions

If this is right

  • The $tq\gamma$ vertex is constrained to effective-coupling combinations below 0.19–0.52 at $\Lambda = 1$ TeV, excluding new-physics parametrizations that predict larger values.
  • The upper limits on $\mathcal{B}(t\to\gamma u)$ and $\mathcal{B}(t\to\gamma c)$ reach $10^{-5}$ to $10^{-4}$, roughly an order of magnitude beyond the previous best limits.
  • FCNC single-top-plus-photon production cross sections are bounded to 33–78 fb, so models predicting production above a few tens of femtobarns are ruled out.
  • The limits complement existing constraints on $tqZ$ flavour-changing couplings, jointly narrowing the allowed flavour structure of dimension-six operators.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • At a coupling near the current limit, a future dataset several times larger should either reveal a $t\gamma$ signal or push the branching-ratio bound below $10^{-5}$, a region where several new-physics models predict observable rates.
  • Because this analysis is optimized for the production mode, a dedicated selection for the decay mode that allows additional jets could improve the sensitivity to the $tc\gamma$ coupling, where the decay mode contributes substantially.
  • The same neural-network-plus-profile-likelihood strategy could be adapted to searches for top-quark flavour-changing couplings to a $Z$ boson or a Higgs boson, whose signal topologies and background compositions are similar.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

1 major / 4 minor

Summary. The paper reports a search for flavour-changing neutral current (FCNC) tqγ couplings using 81 fb−1 of proton-proton collision data at √s = 13 TeV collected with the ATLAS detector. Events with exactly one photon, one electron or muon, one b-tagged jet, and missing transverse momentum define the signal region. A neural network based on ten kinematic variables is used to discriminate signal from background, and a profile likelihood fit to the NN output distributions in the signal region and to the photon pT distributions in two control regions extracts the signal and background normalisations. The data are found to be consistent with the background-only hypothesis, and 95% CL upper limits are set on the effective tqγ couplings, on the FCNC tγ production cross sections, and on the branching ratios B(t→γu) and B(t→γc) for left-handed and right-handed couplings. The paper claims these limits are the most stringent to date.

Significance. If the results are correct, they represent a substantial improvement over previous searches, constraining FCNC top-photon couplings to branching ratios of order 10^-5 and cross sections of order tens of femtobarns. The analysis has several clear strengths: the simultaneous profile likelihood fit treats many correlated systematic uncertainties, the electron and hadron fake backgrounds are estimated with data-driven methods in dedicated control regions, the neural network is validated with cross-validation, and the CLs procedure is used correctly. There is no circularity in the statistical procedure: signal and background normalisations are obtained from the fit, and the limits follow from the fitted signal contribution. The manuscript is concise and generally clear. The main reservation concerns the treatment of the dominant electron-fake background, whose kinematic shape uncertainty is not addressed in the systematic evaluation.

major comments (1)
  1. [Section 6 and Section 9]
minor comments (4)
  1. [Table 2] The notation |C(13)*uW + C(13)*uB| is not defined in the text; please define the operator coefficients and explain the meaning of the parentheses and the star superscript.
  2. [Section 6] The validation region used to check the modelling of the electron-fake kinematics is restricted to EmissT < 30 GeV, while the SR requires EmissT > 30 GeV; a sentence explaining why this validates the SR phase space would strengthen the argument.
  3. [Section 9] The limited number of Monte Carlo events is listed among the largest systematic uncertainties, but the implementation of this uncertainty in the fit (for example, Barlow-Beeston or equivalent) is not described; please clarify.
  4. [Section 5] The statement that events must have 'no further jets' is important for the decay-mode acceptance, which is only 0.45–0.51%; please state explicitly whether the required b-tagged jet counts toward this jet multiplicity or whether 'no further jets' means no additional jets beyond the b-tagged one.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: the FCNC limits follow from a profile-likelihood fit to the signal region and independent control regions, with data-driven fake backgrounds normalised in disjoint control samples and all key adjustments propagated as nuisance parameters.

full rationale

The paper's derivation chain is self-contained against external data and does not reduce any prediction to its inputs. The central result is an observed upper limit obtained from a simultaneous binned profile-likelihood fit to the NN-output distribution in the signal region and the W+gamma+jet and Z+gamma control regions (Section 10). The signal contribution is a free parameter scaled from MC signal samples; it is not defined in terms of the fitted backgrounds or vice versa. The dominant e-to-gamma fake background is normalised using fe->gamma measured in dedicated Z->ee and Z->e-gamma electron-fake regions (Section 6), which do not overlap the SR or CRs, with the validation region used only to inflate the systematic uncertainty. The hadron-fake background uses the data-driven product of three HFR yields with a simulation-derived correlation correction and a conservatively varied 50% systematic (Section 7), again independent of the signal extraction. The photon pT shape correction is tuned in the W/Z+gamma control regions in five bins (Section 4) and is applied with an explicit systematic comparing corrected and uncorrected predictions; it is not fitted to the SR. The normalisations of W/Z+gamma backgrounds are free parameters in the fit, which is a standard treatment and not circular. Citations to prior ATLAS performance papers and to the TopFCNC simulation models provide external calibration and generator inputs rather than the claimed physics result. The 'most stringent to date' claim is a comparative statement against the CMS result and does not load any circular step. No quoted equation or fitted parameter is renamed as a prediction, and no load-bearing uniqueness argument is imported from the authors' prior work. The analysis is therefore not circular.

Assumptions & free parameters 6 free parameters · 4 assumptions · 0 invented entities

The analysis rests on standard collider-physics assumptions: the EFT description of the tqγ vertex, the validity of the Monte Carlo generators, and the extrapolation of data-driven fake estimates to the signal region. The free parameters listed above are normalisations and shape corrections fitted to data or determined from data in control regions, each assigned a systematic uncertainty. No new particles or entities are introduced.

free parameters (6)
  • Photon pT shape correction coefficients = Linear function over five pT bins; exact values not quoted in the paper
    Section 4: MC prediction for W+γ+jets and Z+γ+jets is adjusted to data in five pT bins to correct observed mismodelling; this is a shape-only fit in control regions.
  • W+γ+jets normalisation factor = 1.25 ± 0.09 (for LH tuγ coupling)
    Section 10: free parameter in the profile likelihood fit; determined simultaneously with the signal contribution.
  • Z+γ+jets normalisation factor = 1.12 ± 0.12 (for LH tuγ coupling)
    Section 10: free parameter in the fit, constrained by the Z+γ control region.
  • Hadron-fake correlation correction factor = 0.85 ± 0.14
    Section 7: corrects the product-of-event-count estimate for jet→γ fakes; derived from MC and varied by ±50% as a systematic.
  • Electron-to-photon fake efficiency scale factor = 0.978 ± 0.040
    Section 6: data-to-MC scale factor applied to the simulated e→γ fake background.
  • Hadron-to-photon fake scale factor = 1.7 ± 0.3 (stat) ± 1.0 (syst)
    Section 7: normalisation scale factor for the simulated jet→γ fake background, estimated from data.
assumptions (4)
  • domain assumption The tqγ interaction is described by the dimension-six effective operators O_uB and O_uW with energy scale Λ = 1 TeV
    Section 1 and Table 2: the limits on couplings are interpreted in this EFT; if new physics is not captured by these operators, the numerical limits do not apply.
  • domain assumption The TopFCNC model in MadGraph5_aMC@NLO generates signal kinematics correctly at NLO QCD
    Section 4: signal samples for production and decay modes rely on this model; wrong signal kinematics would bias the neural-network discrimination and efficiency.
  • domain assumption Data-driven fake rates and scale factors measured in control regions extrapolate to the signal region
    Sections 6 and 7: electron and hadron fake rates are measured in dedicated regions and applied to the SR; systematic uncertainties are intended to cover residual differences.
  • domain assumption The Monte Carlo generators (Powheg-Box, Sherpa, MadGraph) model Standard Model backgrounds adequately after the documented corrections
    Section 4: all background predictions depend on these generators and tunes; variations of scales, PDFs, and generators are used to estimate uncertainties.

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Cite this review

Pith. "Pith review of Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment." pith.science (2026). https://pith.science/paper/3ZVK7TE6

@misc{pith2026190808461,
  author       = {Pith},
  title        = {Pith review of: Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb$^-1$ of $pp$ collisions at $\sqrts = 13$ TeV with the ATLAS experiment},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3ZVK7TE6}},
  note         = {Machine review of arXiv:1908.08461}
}
abstract

A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb$^{-1}$ of proton-proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a $b$-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the $tq\gamma$ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC $t\gamma$ production via a left-handed (right-handed) $tu\gamma$ coupling of 36 fb (78 fb) and on the branching ratio for $t\rightarrow \gamma u$ of $2.8\times 10^{-5}$ ($6.1\times 10^{-5}$). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC $t\gamma$ production via a left-handed (right-handed) $tc\gamma$ coupling of 40 fb (33 fb) and on the branching ratio for $t\rightarrow \gamma c$ of $22\times 10^{-5}$ ($18\times 10^{-5}$).

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