Single top quark and rare top quark production at ATLAS and CMS

CMS Collaborations); Willem Verbeke (On behalf of the ATLAS

Single top quark and rare top quark production at ATLAS and CMS

T0 review · 0 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The latest ATLAS and CMS results on single top and rare top production all agree with Standard Model predictions within uncertainties.

desk verdict A faithful, well-organized conference summary of already-published ATLAS and CMS top-quark results; useful snapshot, but nothing new to referee. read the letter →

arxiv 1908.07271 v1 pith:GDB2NYPF submitted 2019-08-20 hep-ex

Willem Verbeke (On behalf of the ATLAS , CMS Collaborations) This is my paper · ORCID

classification hep-ex

keywords topquarksingleproductionrareLHCATLASCMStZqfour-top

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 review compiles the ATLAS and CMS measurements of single top quark production and rare top quark channels from LHC Run 2 and reports that all quoted results are consistent with Standard Model predictions within uncertainties. The combined single-top measurements determine $|f_L^V V_{tb}| = 1.02 \pm 0.04 \text{(exp)} \pm 0.02 \text{(theo)}$, a relative precision of about $3.7\%$. Among the rare channels, CMS observes $tZq$ production at $8.2$ standard deviations with $\sigma = 111^{+11}_{-9}$ fb, reports first evidence for $t\gamma q$ at $4.4$ standard deviations, and measures four-top production at $\sigma(tttt) = 12.6^{+5.8}_{-5.2}$ fb, which yields a 95% C.L. bound $|y_t/y_t^{\mathrm{SM}}| < 1.7$. These processes are sensitive to top couplings and to new physics via unitarity cancellations, so the paper's bottom line is that no deviation from the Standard Model has yet surfaced in this sector.

What carries the argument

The load-bearing machinery is a set of LHC Run 2 analyses built on multivariate discriminants, most often boosted decision trees (BDTs), that separate rare signals from backgrounds. The $tZq$ search uses a BDT-based lepton identification that raises signal efficiency and suppresses nonprompt muons (electrons) by a factor $8$ ($2$). The $t\gamma q$ search extracts its signal from a fit to a BDT distribution. The four-top searches use BDT discriminants combining top-tagging, event topology, hadronic activity, and $b$-tagging, plus a data-driven $t\bar t$ background estimate, and a large-radius reclustered-jet categorization in one channel. For the $t\bar t$/$tWb$ interference study, the discriminating variable is $m_{b\ell}^{\mathrm{minimax}}$, which stays below $\sqrt{m_t^2-m_W^2}$ for parton-level $t\bar t$ and is unfolded to compare interference modeling schemes. These techniques are what allow the rare signals to be extracted from data.

What would settle it

Compare the quoted headline numbers with the cited papers: if the $tZq$ cross section of $111^{+11}_{-9}$ fb, the four-top cross section of $12.6^{+5.8}_{-5.2}$ fb, or the combined $|f_L^V V_{tb}|$ of $1.02 \pm 0.04 \pm 0.02$ does not match the corresponding ATLAS or CMS publication, then the review misrepresents the experiments it summarizes.

Watch

Extended reading notes

Core claim

On its own terms, the paper makes no new measurement; it is a review that reports the latest ATLAS and CMS results on single top and rare top production. CMS measures $|f_L^V V_{tb}| = 1.00 \pm 0.08 \text{(exp)} \pm 0.02 \text{(theo)}$ in the t-channel, and the ATLAS and CMS 7 and 8 TeV combination sharpens this to $|f_L^V V_{tb}| = 1.02 \pm 0.04 \text{(exp)} \pm 0.02 \text{(theo)}$. The $tZq$ process is observed at $8.2$ s.d. with $\sigma = 111^{+11}_{-9}$ fb; $t\gamma q$ is evidenced at $4.4$ s.d.; four-top production is measured at $12.6^{+5.8}_{-5.2}$ fb with an observed significance of $2.6$ s.d.; and the first direct charged-lepton flavor violation search sets $B(t\to\ell\ell'q)<1.86\times10^{-5}$ or $B(t\to e\mu q)<6.6\times10^{-6}$. The paper's assessment is that every result is compatible with Standard Model expectations within uncertainties.

Load-bearing premise

The review's account stands on the quoted numbers being faithful transcriptions of the cited ATLAS and CMS analyses, and on those analyses' background estimates and systematic assignments being sound.

Editorial extensions

If this is right

With $tZq$ observed at $8.2$ s.d., this process becomes an established electroweak top production channel, so future data can test the $tZ$ and $WWZ$ couplings at percent-level precision.
The combined single-top $|f_L^V V_{tb}|$ precision near $3.7\%$ makes $V_{tb}$ the best-determined CKM element inferred from top quarks, tightening CKM unitarity checks.
The $t\gamma q$ evidence at $4.4$ s.d., once confirmed, gives a direct route to the top quark's charge and dipole moments; a $5$ s.d. observation would open anomalous top-coupling measurements.
The four-top cross section near the Standard Model value and the $|y_t/y_t^{\mathrm{SM}}|<1.7$ limit bound new physics in top-Higgs couplings and in $gg\to t\bar t t\bar t$.
The cFLV limits at the $10^{-5}$--$10^{-6}$ level improve on indirect bounds by several orders of magnitude and start to constrain flavor-violating top decays.

Reading between the lines

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

Beyond the paper, the ratio $\sigma(tZq)/\sigma(tW)$ could isolate $tZ$ coupling deviations from PDF and scale uncertainties that cancel in the ratio, making it a sharper new-physics test than either cross section alone.
Beyond the paper, if the four-top cross section drifts upward in the full ATLAS Run 2 combination, the more natural explanation would be new physics in the production vertex rather than a modified Yukawa, because the measured rate falls as $|y_t|$ grows while the background rises.
Beyond the paper, the null cFLV result implies that targeted high-luminosity searches for $t\to e\mu q$ could probe branching fractions down to $10^{-6}$ and below, directly testing TeV-scale lepton-flavor-violating top couplings.

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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

Quick take: this is a proceedings note, not a research paper. It presents no new data, no new analysis, and no new interpretation. What it does well is compile the current ATLAS/CMS results on single top and rare top production into a readable summary with the right numbers and appropriate qualifications. If someone asks for a one-stop update on tZq, tgamma-q, Vtb combination, tttt, and cLFV, this is a place to point them.

The concrete content: CMS's tZq observation (8.2 s.d., 111 ± 13 (stat) +11−9 (syst) fb), first evidence for tgamma-q (4.4 s.d.), combined Vtb determination |f_L V_tb| = 1.02 ± 0.04 ± 0.02, and the CMS four-top cross section 12.6 +5.8 −5.2 fb with |yt/ySM_t| < 1.7 — all match the cited papers as far as I can tell. I checked the headline numbers against the references; I found no transcription error. The text is also careful to distinguish observed from expected significances and to note that the four-top deviation is a fluctuation consistent with the SM.

The soft spots are minor but real. First, the novelty is zero by design; this is a summary. The value is in the selection and wording, not in new knowledge. Second, the paper relies entirely on the fidelity of the original collaborations' analyses; there is no independent cross-check possible within the text. That is structural for a review, not a flaw. Third, typesetting problems — 'A TLAS' in the title, '|fL VVtb|' for what should be |f_L V_tb|, and 'cFLV' for cLFV — are cosmetic but should be fixed. They don't affect the physics.

I won't cite this in my own work; I'll cite the primary papers. But for a reading group or a newcomer, it is a fine orientation. As a peer-review matter: I'd send it to a copyeditor, not a referee. There is nothing to referee beyond the references, and the references are already peer-reviewed. If the venue expects original contributions, desk reject is appropriate; if it publishes conference summaries, accept after minor cleanup.

Referee Report

0 major / 4 minor

Summary. This proceedings paper by W. Verbeke, on behalf of the ATLAS and CMS collaborations, reviews the status of single top quark production and rare top quark processes at the LHC as of August 2019. It summarizes t-channel single top production and the determination of Vtb, including the ATLAS+CMS combination; the observation of tZq production by CMS (sigma = 111 +11-9 fb, 8.2 s.d.); first evidence for t gamma q by CMS (115 +/- 30 fb, 4.4 s.d.); ATLAS and CMS searches for four-top production, including the CMS 137 fb^-1 multilepton result (sigma = 12.6 +5.8-5.2 fb, 2.6 s.d., with |yt/ySM_t| < 1.7); and the first direct ATLAS search for charged lepton flavor violation in top quark decays. The paper is a secondary compilation: it reports rather than derives results, and its value depends on the fidelity of the quoted numbers and references.

Significance. The paper's central claim, that the latest ATLAS and CMS results on these rare processes are presented, is sound. It is a concise, well-referenced snapshot of the field, useful for conference proceedings, and the headline numbers (tZq cross section and significance, t gamma q evidence, combined Vtb, and CMS four-top result) are consistent with the cited primary publications. The paper does not attempt new derivations; consequently the usual review criteria for internal derivations do not apply, and I judge it on whether the reported results faithfully represent the sources. I found no concrete numerical transcription error. The main value is traceability: each result is tied to a public ATLAS/CMS paper or conference note, with uncertainties and significances reported. The main risk, unavoidable in this genre, is notation and rendering fidelity; the garbled notation in Section 2 should be corrected, but it does not affect the substance of the summary.

minor comments (4)

[Section 2, first paragraph] The charge ratio is printed as sigma_{t-ch,t}/sigma_{t-ch,t}, which equals 1 by construction and cannot be 1.66; the denominator should presumably display the anti-top-quark cross section, sigma_{t-ch,anti-t}. Please correct the notation and verify the printed formula against CMS arXiv:1812.10514.
[Title and Section 2] Several rendering artifacts should be fixed: the title contains 'A TLAS' instead of 'ATLAS'; the coupling notation '|fL VVtb|' should be typeset as |f_{LV} V_{tb}|; and 'idenﬁciation' should be 'identification'.
[Section 3, four-top discussion] The sentence 'the cross section is proportional to |yt|4' followed by a statement that the measured cross section decreases as a function of |yt| is potentially confusing: the two statements refer to different quantities. Please rephrase to distinguish the expected tttt signal yield from the fitted cross section in the presence of a growing ttH background.
[Figures] Figure 1 contains three panels from three different analyses, and the text refers to them only by the overall figure number. Adding explicit panel labels (e.g., left, middle, right, or a,b,c) in both captions and text would improve readability.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the paper is a conference summary reporting external ATLAS and CMS measurements, with no fitted inputs called predictions and no load-bearing self-citation chain.

full rationale

The paper makes no derivation claim of its own. Its abstract states that it presents the latest ATLAS and CMS results on single top quark and rare top quark production, and every numerical result is explicitly attributed to an external experimental publication or conference note (e.g., CMS PRL 122, 132003 for tZq; CMS PRL 121, 221802 for t-gamma-q; ATLAS+CMS arXiv:1902.07158 for the Vtb combination; CMS-PAS-TOP-18-003 for four-top). None of the quoted quantities is fitted within this paper, and no parameter is renamed as a prediction. The only self-referential aspect is that the author is a member of the collaborations whose results are summarized, but the cited analyses are independent experimental measurements with their own data, systematics, and statistical procedures; the summary does not rely on an unverified uniqueness theorem or on a prior work of this author to justify its central content. The correctness of the review depends on faithful transcription of the cited numbers, which is a reporting assumption rather than a circular one. Under the review rules, a paper that is self-contained against external benchmarks receives score 0-2; here the external anchoring is direct and complete, so the appropriate finding is no significant circularity (score 0).

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

This review introduces no free parameters and no invented entities. Its content is a summary, so the only assumptions are trust in the original experimental analyses, the fidelity of the author's transcription, and the reliability of quoted theory predictions. No fitting, prediction, or derivation is performed in this paper.

assumptions (3)

domain assumption The cited ATLAS and CMS analyses were performed correctly and their quoted uncertainties are accurate.
The review's statements consist entirely of summaries of these cited works; any error in them propagates to this paper.
domain assumption The numerical values quoted in this summary are faithful transcriptions of the original papers.
Visible rendering artifacts in the arXiv text, such as 'fL VVtb', suggest that proofreading is imperfect, so transcription fidelity cannot be taken for granted.
domain assumption The Standard Model and Monte Carlo predictions used for comparison, from POWHEG and aMC@NLO, are reliable for the observables shown.
Agreement with the Standard Model is claimed for tZq and for the t-channel polarization asymmetry; this depends on generator predictions the review does not independently validate.

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Cite this review

Pith. "Pith review of Single top quark and rare top quark production at ATLAS and CMS." pith.science (2026). https://pith.science/paper/GDB2NYPF

@misc{pith2026190807271,
  author       = {Pith},
  title        = {Pith review of: Single top quark and rare top quark production at ATLAS and CMS},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/GDB2NYPF}},
  note         = {Machine review of arXiv:1908.07271}
}

read the original abstract

The latest results from ATLAS and CMS on single top quark production and rare production channels of top quarks at the LHC are presented.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

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