Bounding exotic top decays inclusively at the FCC-ee

Barbara Mele; Dibyashree Sengupta; Gennaro Corcella

REVIEW 2 major objections 2 minor 1 cited by

Bounding exotic top decays inclusively at the FCC-ee

T0 review · 2 major / 2 minor · reviewed 2026-08-05 · deepseek-v4-flash

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Pith's one-line read Measuring the total top decay width at FCC-ee can bound any exotic top decay mode, without knowing what the mode is.

desk verdict The inclusive top-width excess idea at FCC-ee is useful and should be refereed, but the abstract's 'model-independent branching fraction' claim is too strong without an explicit assumption about the dominant t→Wb width; also the supplied full text is the wrong paper, so no audit is possible. read the letter →

arxiv 2508.06954 v1 pith:ZZEU6TMF submitted 2025-08-09 hep-ph hep-ex

Gennaro Corcella , Barbara Mele , Dibyashree Sengupta This is my paper

classification hep-phhep-ex

keywords topquarkexoticdecaystotalwidthFCC-eee+e-collisionsmodel-independentboundsrarepairproduction

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 proposes using direct top-pair production at a future e+e- collider, FCC-ee, to measure the total width of the top quark and compare it to the standard-model prediction. Because any non-standard decay of the top contributes to the total width, an observed excess would be a model-independent signal of new physics, regardless of the exotic final state. The authors argue that the clean FCC-ee environment makes this inclusive measurement feasible, allowing rare top-quark branching fractions to be constrained more directly than in hadronic collisions. If correct, this would establish a new, final-state-agnostic strategy for probing new physics coupled to the top quark.

What carries the argument

The central object is the inclusive top decay width, $\Gamma_t$, reconstructed from direct $t\bar t$ production at FCC-ee. Its measured value is compared with the precisely predicted standard-model total width $\Gamma_{\mathrm{SM}}$; the difference, $\Gamma_t - \Gamma_{\mathrm{SM}}$, equals the sum of all non-standard partial widths and therefore bounds any individual exotic branching fraction. The method works because it needs no specific decay-mode tag, only an accurate total-width measurement.

What would settle it

A concrete check is to extract the top total width from two different final states at FCC-ee, for example leptonic and hadronic top decays. If the two reconstructed widths disagree beyond combined uncertainties, the inclusive width is not a clean production-plus-decay observable, undermining the model-independent bound. A second check: if the measured t tbar cross section at FCC-ee deviates from the standard-model prediction by an amount comparable to the claimed width sensitivity, the normalization assumption behind the method would fail.

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Extended reading notes

Core claim

The central claim is that the total top decay width measured in e+e- -> t tbar events at FCC-ee provides an inclusive, model-independent probe of exotic top decays. In the standard model the top width is predicted with good accuracy; any measured excess over that prediction must come from additional, non-standard partial widths. Summing all possible exotic partial widths, the excess directly bounds the branching fraction of any rare top decay, whether or not the decay products are identified. The method exploits the clean lepton-collider environment, where the t tbar production rate and final-state selection are well understood, so that the total width can be extracted without relying on a s

Load-bearing premise

The argument assumes the t tbar production rate and event selection are essentially insensitive to how the top decays, so any measured width excess is a true signal of new decay modes rather than an artifact of detection efficiency or luminosity error.

Editorial extensions

If this is right

Any beyond-standard-model top decay, whatever its final state, contributes to the measured total width, making the total-width check model-independent.
The clean e+e- environment at FCC-ee can resolve a width excess that would be masked by QCD backgrounds and combinatorics at hadron colliders.
A measured excess translates directly into an upper limit on the branching fraction of rare or exotic top decays.
The method also constrains new-physics models that modify the top width through loops or new decay channels, even when no individual exotic final state is searched for.

Reading between the lines

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

The same inclusive-width logic could be transferred to other proposed lepton colliders and to other short-lived particles, such as the Higgs boson, where a total-width measurement can bound exotic and invisible decays without tagging them.
The practical sensitivity will likely be limited by luminosity and beam-energy uncertainties rather than by detector acceptance; a dedicated differential study of t tbar production shapes could separate production systematics from decay-width effects.
A combined analysis of the total width and specific final-state searches would allow the model-independent excess to be decomposed into contributions from multiple exotic modes, sharpening the physics reach beyond a single inclusive limit.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Colleague,

Two things you should know upfront. First, the full text attached to this review is not the paper — it's an unrelated cs.AI benchmark (arXiv:2508.06960). So I can only judge the abstract of 2508.06954, and anything I say about the body is guesswork. Second, the central idea is sound but the abstract's phrasing is too strong.

The inclusive strategy — measure the total top width in e+e- → ttbar and compare to the SM prediction — is a clean way to bound non-SM decay contributions without knowing the final state. That's a genuinely useful complement to LHC channel-by-channel searches, and it strengthens the FCC-ee physics case. If the body contains a proper simulation with an error budget, it could be a solid reference for the machine's top program.

The soft spot is the step from 'width excess' to 'limits on rare decay branching fractions.' A measured excess constrains the total non-SM width, not specifically exotic branching fractions. If new physics also changes the dominant t→Wb partial width — say, suppresses it while adding an exotic channel — the two effects can cancel, leaving no excess. The stress-test note is right about this. The paper can fix it by either justifying the assumption that t→Wb is SM or by reframing the result as a bound on the total non-SM width, with branching-fraction limits derived under a stated benchmark. That's a presentational fix, not a fatal flaw, and the body may already address it.

The abstract alone has no numbers, no detector assumptions, no error budget, so I can't audit soundness or novelty here. The reference list isn't inspectable, so I can't check prior ILC and FCC-ee width-extraction studies. That's not a knock on the authors; it's a limitation of the material I was given.

Who's this for? People working on FCC-ee physics potential and top-quark EFT interpretations. It's a prospective sensitivity projection, not a constraint on today's data, and it should be labeled that way.

Recommendation: send it to peer review. The idea is worth refereeing, and the caveat is fixable. I'd ask the referee to push on the t→Wb assumption and the interpretation of the bound. For a reading group, wait until you can get the actual paper.

Best, [You]

Referee Report

2 major / 2 minor

Summary. The abstract claims that by studying direct t-tbar production at the FCC-ee and measuring the inclusive top-quark width, one can detect any exotic decay contribution and establish model-independent limits on rare top-quark branching fractions. The argument is that the measured total width, compared with the accurately known standard-model prediction, bounds any non-SM partial width regardless of the final state. However, the supplied full text is not this paper; it is an unrelated manuscript on dataset discovery. Thus no derivations, simulations, systematic uncertainties, or numerical results are available to support the abstract's claims.

Significance. If the proposed method works, it would be a valuable complement to direct searches for rare top decays at a future e+e- collider. The inclusive-width approach could in principle be model-independent with respect to the exotic final state. However, the manuscript as submitted cannot be evaluated: the body text is a different paper. Within the abstract alone, the central model-independence claim also requires an unstated assumption about the dominant t→Wb partial width. These issues prevent the current submission from being accepted or meaningfully revised.

major comments (2)

[Full text] The full text of the submission is not the paper described in the abstract. It is a paper on dataset discovery, not a study of top-quark physics at FCC-ee. No equations, simulations, detector assumptions, error budget, or numerical results are present for the claimed method. This makes the manuscript impossible to audit and is a load-bearing deficiency.
[Abstract] The abstract states that the method establishes 'model-independent limits for rare decays branching fractions of the top quark.' As written, the comparison Γ_t^meas = Γ_SM + Γ_BSM only constrains the total non-SM contribution to the width. If new physics also modifies the dominant t→Wb partial width, then Γ_BSM includes both that modification and any exotic partial width. A positive exotic branching fraction can be compensated by a reduced t→Wb width, leaving Γ_t^meas ≈ Γ_SM. Conversely, an enhanced t→Wb coupling would mimic an exotic excess. The claim of model-independent rare-decay bounds therefore requires the assumption that Γ(t→Wb) is unchanged; this must be stated and justified, or the claim must be weakened to a bound on the total non-SM width contribution.

minor comments (2)

[Abstract] The abstract does not specify the FCC-ee center-of-mass energy, integrated luminosity, or the expected precision of the total-width extraction. These are necessary for any quantitative statement and would be expected in the body.
[Abstract] The phrase 'model-independent limits for rare decays branching fractions' contains a grammatical redundancy; consider 'model-independent bounds on rare top-quark branching fractions.'

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity identified: the method is an excess-over-SM-width comparison with external theory input.

full rationale

The only coherent text available is the abstract. The proposed strategy measures (or would measure) the inclusive top width in ttbar production at FCC-ee and compares it with the Standard Model prediction; any excess is interpreted as the sum of non-SM partial widths and converted into limits on rare branching fractions. This is not circular: the SM width is an independent external input, and the exotic excess is defined as the difference between measured and SM widths. No parameter is fitted to a subset of the target data and then renamed a prediction, and no load-bearing self-citation appears in the abstract. The body text supplied is badly garbled and cannot support a quoted reduction of any equation to its inputs. The known caveat that new physics could also modify the dominant t->Wb partial width, so that an unchanged total width need not imply zero exotic branching, is an assumption about model-independence, not a circularity of the derivation; it belongs to a correctness risk assessment rather than to a circularity score. Accordingly no circular step is exhibited, and the score is 0.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

Only the abstract was available and the supplied body is a different paper, so a complete audit is impossible. The assumptions listed are those minimally required for the inclusive strategy; no free parameters or invented entities could be identified from the abstract. Any tuned simulation inputs or fitted detector parameters in the body would add to this ledger.

assumptions (3)

domain assumption Top-pair production at e+e- is independent of the top decay modes (narrow-width approximation).
Needed to convert a total-width excess into an exotic branching-fraction bound; implicit in the abstract's inclusive strategy.
domain assumption The standard-model top width is an accurate external input.
The abstract frames the measurement as bounding 'exotic excesses', i.e., deviations from the SM expectation; the SM width must be known to comparable or better precision.
domain assumption FCC-ee luminosity and detector performance allow the required width precision.
The abstract assumes the 'clean' environment is sufficient; the reach of the bound depends on this.

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Cite this review

Pith. "Pith review of Bounding exotic top decays inclusively at the FCC-ee." pith.science (2026). https://pith.science/paper/ZZEU6TMF

@misc{pith2026250806954,
  author       = {Pith},
  title        = {Pith review of: Bounding exotic top decays inclusively at the FCC-ee},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZZEU6TMF}},
  note         = {Machine review of arXiv:2508.06954}
}

abstract

Since its discovery, the top quark has never been produced and studied in an environment as clean as that predicted for $e^+e^-$ collisions at future colliders. Details of the top quark's properties, completely unattainable in hadronic collisions, can be analyzed via lepton collisions. New strategies for analyzing the physics of the top quark can, therefore, be developed in such a spectacularly clean environment. Here we focus on the possibility of inclusively measuring exotic excesses in the top decay width by studying the direct production of $t\bar t$ at the FCC-$ee$, thus establishing model-independent limits for rare decays branching fractions of the top quark.

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Reviewed August 5, 2026 · model on record in the stance chip above.

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