Top flavour-changing neutral interactions: theoretical expectations and experimental detection

J. A. Aguilar-Saavedra

REVIEW

Top flavour-changing neutral interactions: theoretical expectations and experimental detection

J. A. Aguilar-Saavedra

High Energy Physics - Phenomenology · Submitted 29 Sep 2004 · 49 references

Pith's verdict: A competent, honest 2004 review whose real product is the reach tables; the soft spots are the usual ones for a review of this kind and the paper mostly flags them itself.

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arXiv IDhep-ph/0409342 (version 4)

Pith IDEWOAE6BV, Pith's permanent record of this paper and its review

CitationJ. A. Aguilar-Saavedra. "Top flavour-changing neutral interactions: theoretical expectations and experimental detection." arXiv:hep-ph/0409342 (2004). https://pith.science/paper/EWOAE6BV

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Editor's note

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Bottom line: this is a useful reference compilation, not a new derivation, and it knows it. The abstract says 'we review' [abstract:s1] and the body delivers exactly that. If you need the operator basis, the SM loop numbers, and the LHC/LC reach targets in one place, this is the paper to cite, and it earned its citations honestly. I'd send it out without much hesitation, but I'd tell the referee to focus on the tables, not the prose, because the tables are the only original quantitative content and they carry the whole observability claim [abstract:s0]. The effective Lagrangian in Eq. (1) is the conventional basis, the SM branching ratios in Eq. (5) and Table 1 are correctly attributed to the published loop calculations, and the GIM/b-mass story in Eq. (3) is right. The model survey in Section 3 is a fair summary of what several groups had found by 2004, and the benchmark couplings (Xqt ~ 0.015 for a Q=2/3 singlet, gct ~ 0.20 for leptoquarks) are presented as order-of-magnitude estimates, which is what they are. The soft spot I'd actually press on is the one the stress-test names: the 3-sigma reach in Tables 5-7 rests on a stated but unquantified background assumption (B <= 5 events after cuts, statistical errors only, no NLO). The paper does say the cross sections are parton-level and that a detailed simulation is not attempted, so this is acknowledged rather than hidden, but the tables are quoted to two significant figures as if the acceptance and background modelling were under control. Since the reach in coupling scales as B^(1/4), a background an order of magnitude larger than assumed moves the quoted limits by a factor of ~1.8, and the paper's own remark that SM single top has a much larger cross section than the signal makes that the crux. I would not, however, weight the t->qgamma discrepancy the stress-test raises: the 3.4e-5 number is a cited CMS detector-level result from ref. [1], not an internal inconsistency with the paper's own parton-level 1.0e-5, and the paper is transparent that its numbers are parton-level. The reader's weakest-assumption point about B-physics and EWPO constraints on the benchmark couplings is fair in principle, but the paper does say 'present experimental data allow Xqt ~ 0.015' for the singlet case, so it is not entirely unchecked. For a 2004 review that is acceptable. Who should read it: phenomenologists setting FCNC search targets, experimentalists writing reach projections, and students who want the operator basis and the SM numbers in one place. It deserves a serious referee, mainly to check the background assumptions behind Tables 5-7 and the consistency of the quoted limits.

Referee report

Significance

The review is a useful and durable reference: it puts the Standard Model rates, the model expectations, and the collider reach in one place, in a common operator basis, with a consistent set of conventions. The reach tables define concrete targets for experimental searches, and the observation that single top production gq → t is the most sensitive channel for the gluon operator has been borne out by subsequent experimental practice. The paper is honest about what it does and does not compute: it states the discovery criterion used for the reach estimates and it flags that a full simulation of signal and background is required. The value of the paper rests on the reach compilation rather than on any new derivation, and that is appropriate for a review.

Major objections (3)

[Tables 5–7 and surrounding text] The 3σ limits in Tables 5–7 rest on the criterion S/√B = 3 with B ≤ 5 events after cuts, but the expected background event counts per channel are not tabulated anywhere in the manuscript. The text itself notes that the Standard Model single-top background to gq → t has a much larger cross section than the signal, which makes the assumed background level the crux of the reach estimate rather than a detail. Because the coupling reach scales as B^(1/4), a background a hundred times larger than assumed degrades the quoted coupling limits by roughly a factor of three. The manuscript should state, for each channel in Tables 5–7, the background level assumed after cuts, give the scaling of the quoted limits with that level, and state explicitly that these are parton-level leading-order estimates without detector simulation. The authors already acknowledge that a detailed simulation is required; the request is to make the dependence of the quoted numbers on the unstated background assumption visible to the reader.
[Section 3 and Table 2 versus Tables 5–7] The abstract states that the couplings 'can reach observable levels in many of its extensions', but the manuscript does not make explicit, model by model, whether the rates collected in Table 2 fall above or below the reach quoted in Tables 5–7. For the most motivated cases, the two-Higgs-doublet and MSSM entries in Table 2 appear to sit at or below the LHC reach, so the reader is left to perform the comparison that decides the scope of the claim. A short column in Table 2, or a sentence per model in Section 3, stating whether the predicted rate is within reach of the corresponding entry in Tables 5–7 would make the central claim precise and would let the reader see which extensions the 'observable levels' statement actually covers.
[Section 3, benchmark couplings] The benchmark coupling sizes used to argue that the rates are observable, for example Xqt ≈ 0.015 in the Q = 2/3 singlet-quark model and gct ≈ 0.20 in the scalar-leptoquark model, are quoted as order-of-magnitude model estimates without a systematic statement of the existing constraints from B physics, K mixing, and electroweak precision data. Where such constraints are already given in the cited literature, a short summary would let the reader judge whether the benchmarks survive; where they are not, the manuscript should say so. This matters because the 'observable levels' claim is only as strong as the benchmark values, even though the model-independent reach tables remain valid regardless.

Minor objections (7)

[Table 3] The leading-order cross sections in Table 3 should specify the parton distribution set, the factorization and renormalization scales, and the collider energy assumed. Since the coupling reach scales as the inverse square root of the cross section, a 30% cross-section uncertainty propagates to roughly a 15% uncertainty on the quoted coupling limits, which is worth stating.
[Eq. (1)] The effective Lagrangian should state the sign convention for σμν and the hermiticity and CP properties of the form factors. The text notes that for an on-shell Z the γμ and σμν terms are not independent; this should also appear in the equation's caption so that the reader does not double-count the two operator choices when comparing Tables 5 and 6.
[Eq. (2) and Table 1] The numerical value Γ(t → bW+) = 1.61 GeV is obtained with mt = 178.0 GeV. The manuscript should state that the entries in Table 1 scale with mt, so that a reader using a different top mass can rescale them.
[Section 2, discussion of the dipole operators] The statement that the dipole terms 'do not appear in the Standard Model' should read 'do not appear at tree level'; they are generated at one loop, which is precisely what Eq. (3) computes. As written the sentence can be misread as contradicting the loop calculation that follows.
[Table 2] The caption of Table 2 should state the model parameters assumed for each column (tan β, the Higgs and superpartner masses, the leptoquark mass, and so on), since the entries are not meaningful without them.
[Tables 5–7] The captions should state clearly that Tables 5 and 6 quote limits on couplings while Table 7 quotes limits on branching ratios obtained through the conversion of Table 1, and should state the collider energy and integrated luminosity assumed for each table.
[Section 4, photon-photon option] The remark that the process γγ → tc can constrain the γtc vertex should be accompanied by the corresponding sensitivity estimate, or by an explicit statement that it is not competitive with the other channels, so that the reader can place it relative to Tables 5–7.

How Pith checks papers → · Full audit of this paper →

The paper

The paper claims

Top quarks may change flavour far more often than the Standard Model allows, and this review maps how large those couplings could be and which LHC and linear-collider signals would reveal them.

The reading

This review asks one question: how large can a top quark's flavour-changing neutral couplings to a $Z$ boson, a photon, a gluon or a Higgs be, and can the LHC or a high-energy $e^+e^-$ collider see them? In the Standard Model the answer is essentially no. Loop-induced rates give branching ratios of about $10^{-14}$ for $t^3$ and $t^3$, and $10^{-12}$ for $t^3$, because the GIM mechanism cancels the heavy-quark contributions almost exactly. The author's case is that many well-motivated extensions push these couplings to branching ratios of $10^{-4}$ to $10^{-3}$ — four to ten orders of magnitude higher — and that the resulting anomalous single-top processes $gq^3$, $^3t$, $t$ and $Ht$ at the LHC have cross sections large enough to be pulled out of the top-pair background. If that holds, the LHC has a real window on new physics that ordinary top-pair counting cannot open, and the tabulated three-$\sigma$ limits define the target.

What carries the argument

The argument is carried by a general effective Lagrangian written in terms of four gauge- and Higgs-boson vertices, each split into a vector piece and a magnetic-dipole piece: $x_{qt}$ for $Ztq$, $^3_{qt}$ for $^3tq$, $^3_{qt}$ for $gtq$ and $g_{qt}$ for $Htq$, with the couplings normalised so that $|x^L_{qt}|^2+|x^R_{qt}|^2=1$ and similarly for the others. That normalisation is what lets the author compare models on a single footing: a model either saturates the coupling or it does not, and the branching ratio follows from the decay-width formulas the paper collects. The loop functions do the suppressing work in the Standard Model — the relevant combination for $t^3$ is the GIM-difference $f'^3_V(m_b^2/M_W^2)$, numerically about $-9.1 imes 10^{-9} - 4.7 imes 10^{-9}i$, which is why the rate collapses. On the detection side the machinery is a table of single-top cross sections at the LHC per unit coupling squared — for example $(9.0+2.6) imes 10^5,|^3_{ut}|^2$ picobarns for $gu^3$ — converted into three-$\sigma$ limits on branching ratios, supplemented by the corresponding numbers for $e^+e^-^3$ and $e^3$ at a linear collider.

Extended reading notes

Core claim

The paper's central claim is a two-sided accounting. On one side it collects the Standard Model predictions for top flavour-changing neutral decays and shows they are hopelessly small: $t^3$ at $4.6 imes 10^{-14}$, $t^3$ at $4.6 imes 10^{-12}$, $t^3$ near $10^{-14}$ and $t^3$ near $3 imes 10^{-15}$, the last two suppressed by the small bottom mass and the GIM cancellation in the loop function. On the other side it surveys extensions — models with an extra $Q=2/3$ singlet quark, scalar leptoquarks, topcolour, two-Higgs-doublet and supersymmetric schemes, warped extra dimensions, little-Higgs models with $T$ parity — and finds branching ratios in the $10^{-7}$ to $10^{-3}$ range, with the gluonic dipole $t^3$ typically the largest. The conclusion the author draws is that these couplings are not merely a curiosity of precision physics: anomalous single-top production at the LHC is sensitive to them at the level the models predict, so a null search converts directly into meaningful constraints on new physics, and a signal would be unambiguous because the Standard Model background is effectively zero.

Load-bearing premise

The claim that these couplings are observable rests on benchmark sizes taken from specific models at face value, without a systematic check of whether the same new physics is already squeezed by $B$-physics, kaon-mixing or electroweak-precision measurements.

Editorial extensions

If this is right

Because the Standard Model rates sit near $10^{-14}$, any observation of $t^3$ or $t^3$ above that level is new physics with no Standard Model background to subtract, which makes even a handful of events meaningful.
Anomalous single-top production $gq^3$ has the largest cross section per unit coupling of the four vertices, so it is the most sensitive channel to the gluonic dipole coupling $^3_{qt}$ and the natural place to look first.
If couplings sit at the benchmark values the review adopts, the LHC three-sigma sensitivities of roughly $10^{-4}$ in $t^3$ and $10^{-5}$ in $t^3$ line up with the predicted rates, so the search is matched to the models rather than far from them.
A high-energy $e^+e^-$ linear collider adds $e^+e^-^3$ and $e^3$, probing the same vertices with different systematics and with polarised beams that the paper shows can be used to separate coupling structures.
The tabulated limits are model-independent: even if a particular extension falls away, the reach numbers remain a statement about how well the LHC can constrain any new physics entering these four vertices.

Reading between the lines

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

A measured rate well above $10^{-14}$ but well below $10^{-4}$ would point to a moderately high new-physics scale rather than an order-one coupling, so the shape of the exclusion curve carries as much information as the limit itself.
The gluonic dipole also enters $t\bar t$ production with one off-shell top, so single-top and pair-production searches are not independent measurements of the same quantity; separating the gluon dipole from the $Z$ and photon ones would need a joint fit.
Flavour constraints from $B$ physics, $K$ mixing and electroweak precision act on the same operators in specific models and could push the benchmark couplings down, so a model-by-model confrontation would sharpen which rows of the summary table are still viable.
Polarised beams, at the $P_{e^-}=0.8$ and $P_{e^+}=-0.6$ values the paper quotes, turn the $tq$ final state into a spin analyser that could in principle distinguish vector from dipole couplings rather than merely counting events.

Original abstract

Top flavour-changing neutral interactions with a light quark q=u,c and a gauge or Higgs boson are very suppressed within the Standard Model (SM), but can reach observable levels in many of its extensions. We review the possible size of the effective vertices Ztq, gamma tq, gtq and Htq in several SM extensions, and discuss the processes in which these interactions might show up at LHC and at a high energy e+ e- linear collider.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 2,366 input tokens, $0.0001 reported.

What the abstract itself states, as probabilities: claims first 0.02 · states limitation 0.11 · human participants 0.02 · controlled comparison 0.12 · randomized assignment 0.01 · states quantitative result 0.07 · compares with prior results 0.20 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v4. Results and the main claim are underlined; show every role.

Top flavour-changing neutral interactions with a light quark q=u,c and a gauge or Higgs boson are very suppressed within the Standard Model (SM), but can reach observable levels in many of its extensions. We review the possible size of the effective vertices Ztq, gamma tq, gtq and Htq in several SM extensions, and discuss the processes in which these interactions might show up at LHC and at a high energy e+ e- linear collider.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	Top flavour-changing neutral interactions with a light quark q=u,c and a gauge or Higgs boson are very suppressed within the Standard Model (SM), but can reach observable levels in many of its extensions.	background 1.00	0.05	0.25	asserted 0.85
s1	We review the possible size of the effective vertices Ztq, gamma tq, gtq and Htq in several SM extensions, and discuss the processes in which these interactions might show up at LHC and at a high energy e+ e- linear collider.	contribution 1.00	0.73	0.14	speculative 0.44

What kind of paper the classifier reports this is: review 1.00 · evidence from literature 0.85.

What was examined, and what was not

sources read

arXiv listing abstract v4

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References

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How the literature uses it

84 arXiv papers cite this in 123 sentences

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Papers citing it · 83 sentences without a confident label

Uses its data or values 10 papers

Within the SM, this suppression leads to the following branching ratios: $BR(t →γc) ∼10^-14$, $BR(t →gc ) ∼10^-12$, $BR(t →Z c) ∼10^-14$ and $BR(t →h c ) ∼10^-15$ [ref] , rendering them unobservable at the current and future colliders.

Rare top quark decays at a 100 TeV proton-proton collider: $t \rightarrow bWZ$ and $t\rightarrow hc$ · Introduction · 0.90
The LHC discovery limit for the $t→ch$ decay process is $BR≥5.8·10^-5$ [ref] for an integrated luminosity of $100fb^-1$.

Top quark rare decays in a two Higgs doublet model for the top · Rare processes and the $t→ch$ and $h→t̅c$ rare decays · 0.98
The experimental limits on these decays are [ref] [equation] The SM predictions for these flavour-changing decays are well below experimental bounds, $Br(t→uZ)_SM∼10^-16$, $Br(t→uH)_SM∼10^-17$, $Br(t→cZ)_SM∼10^-14$ and $Br(t→cH)_SM∼10^-15$ [ref] .

The remarkable role of the vector-like quark doublet in the Cabibbo angle and $W$-mass anomalies · Top decays · 0.90

Cites it as support 1 paper

Clearly the exact value of this decay width, as well as the other flavour violating decays of the top quark, is highly sensitive to the bottom quark mass, as has been pointed out in [ref] .

Rare Top Decays in Minimal and Non-minimal Universal Extra Dimension · $t →cγ$ · 0.99

Cites it as background 26 papers

The FCN couplings of the top quark can be parametrized using either fully gauge-symmetric dimension-six operators [ref] or dimension-four and dimension-five operators in the electroweak broken phase [ref] .

Automatic computations at next-to-leading order in QCD for top-quark flavor-changing neutral processes · Framework · 0.89
Among all the possible decay modes, the flavour-changing neutral-current (FCNC) decays of the top quark, which are absent at tree level and suppressed at loop level by the Glashow-Iliopoulos-Maiani (GIM) mechanism [ref] in the SM, are very sensitive to contributions from new physics (NP) [ref] .

Rare top-quark decays $t \to cg(g)$ in the aligned two-Higgs-doublet model · Introduction · 0.88
Studies have been presented at linear colliders [ref] , lepton-hadron colliders [ref] , as well as hadron colliders [ref] , see also reference there in.

Probe Anomalous $tq\gamma$ couplings through Single Top Photoproduction at the LHC · Introduction · 0.82

Discussion

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