REVIEW 3 major objections 5 minor 9 references
First simultaneous measurement of single and pair production of top quarks in association with a Z boson at the LHC
T0 review · 3 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read The first simultaneous measurement of single and pair top-quark production with a Z boson reports sigma(ttZ+tWZ) = 1.14 +/- 0.07 pb and sigma(tZq) = 0.81 +/- 0.10 pb.
desk verdict The simultaneous tZq/ttZ+tWZ extraction is a genuine first, but the tZq cross section in Eq. (1) contradicts the paper's own fit by about a factor of 8.7; use the companion preprint and correct the proceedings. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
A multiclass deep neural network (DNN) is the central classifier: it takes 26 kinematic and multiplicity variables and assigns each event to one of three output nodes, $t\bar{t}Z+tWZ$, $tZq$, and background, with events placed in the category of highest score. The output distributions are used in a profile likelihood fit with two parameters of interest, one for $\sigma(t\bar{t}Z+tWZ)$ and one for $\sigma(tZq)$; for differential cross sections the same two-parameter fit is performed bin by bin and unfolded with response matrices. Nonprompt leptons, a major background, are estimated with a transfer-factor method in which a misidentification rate measured in a background-enriched region is applied to the signal region. The two-dimensional likelihood scan over the two cross-section ratios to the standard model yields the inclusive results.
What would settle it
In the sideband used to validate the nonprompt estimate, $|m(\ell\ell)-m(Z)| < 20$ GeV, compare the predicted nonprompt yield from the transfer-factor method with the observed event count; a disagreement beyond the quoted uncertainties would mean the background model is biased and the central cross sections are not reliable. Re-running the simultaneous fit with an independent nonprompt estimator, such as a matrix method, and checking whether the two cross sections move by more than the uncertainties would settle the point.
Extended reading notes
Core claim
The central result is that $t\bar{t}Z+tWZ$ and $tZq$ production can be measured together in the three-lepton final state and separated with a multiclass deep neural network. The inclusive cross sections are $\sigma(t\bar{t}Z+tWZ) = 1.14 \pm 0.05\,(\text{stat}) \pm 0.04\,(\text{syst})$ pb and $\sigma(tZq) = 0.81 \pm 0.07\,(\text{stat}) \pm 0.06\,(\text{syst})$ pb within the $70 < m_{\ell^+\ell^-} < 110$ GeV window. The differential cross sections, unfolded to parton level, agree with standard model predictions within uncertainties, with one exception: the $t\bar{t}Z+tWZ$ distribution as a function of $p_T(\ell_W)$ shows a discrepancy at low $p_T(\ell_W)$ that the authors note is reminiscent of a similar trend in inclusive $t\bar{t}$ production. The paper presents this as the first consistent simultaneous constraint on single and pair top+Z production, which sharpens the test of top-$Z$ couplings.
Load-bearing premise
The measurement assumes the lepton misidentification rate measured in a background-enriched region transfers unchanged to the high-multiplicity signal region; if that transfer fails, both the $t\bar{t}Z+tWZ$ and $tZq$ cross sections shift away from their true values.
Editorial extensions
If this is right
- A single simultaneous fit now constrains $t\bar{t}Z$, $tWZ$, and $tZq$ cross sections, reducing the model dependence that separate measurements carry.
- The measured inclusive cross sections are consistent with standard model predictions, so these channels do not, by themselves, point to new physics in top-$Z$ couplings.
- The normalized differential cross sections for five observables provide reference spectra that future standard model and new-physics calculations can be checked against.
- The low-$p_T(\ell_W)$ discrepancy in $t\bar{t}Z+tWZ$ mirrors a known trend in inclusive $t\bar{t}$ production, suggesting the origin may be shared QCD modeling of top-pair production rather than a new-physics effect.
Reading between the lines
- The simultaneous-fit strategy could extend naturally to other associated top-quark productions, such as $tH$ or four-top production, wherever two signal processes share a final state and would otherwise be measured in separate analyses.
- If the low-$p_T(\ell_W)$ trend persists in a future larger dataset, it would strengthen the case that higher-order QCD corrections to $t\bar{t}$+jets also improve $t\bar{t}Z$ modeling; the paper does not claim this.
- A direct closure test of the transfer-factor method in a high-multiplicity validation region, comparing predicted and observed nonprompt yields, would quantify how much of the central values rest on that assumption.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This proceedings paper describes a CMS Run 2 measurement at 13 TeV with 138 fb^-1 of a simultaneous extraction of single-top (tZq) and pair-production (ttZ+tWZ) cross sections in final states with three leptons plus a Z candidate. Events are classified with a multiclass DNN into tZq, ttZ+tWZ, and background categories; inclusive cross sections are obtained from a two-parameter profile likelihood scan, and normalized differential cross sections are presented as functions of five observables. The paper reports sigma(ttZ+tWZ) = 1.14 +/- 0.07 pb and sigma(tZq) = 0.81 +/- 0.10 pb and claims good agreement with the SM except for a low-pT(W) trend in the ttZ+tWZ differential distribution.
Significance. If correct, this would be the first simultaneous measurement of tZq and ttZ+tWZ at the LHC and would provide a useful cross-check of separate CMS results, with a reduced dependence on signal modeling assumptions through the simultaneous fit. The proceedings is concise and directs the reader to the companion paper for technical details, which is appropriate for this format. The paper also quotes SM predictions and prior CMS measurements explicitly. However, as written, the central tZq number is not internally consistent with the quoted SM prediction and measured cross-section ratio, so the result cannot currently be used without a correction.
major comments (3)
- [Section 3, Eq. (1), and Section 4] The quoted tZq cross section is inconsistent with the paper's own inputs. Section 3 gives the SM tZq prediction as 94.2 +/- 3.1 fb and reports the best-fit cross-section ratio as 0.99 +/- 0.13; these imply a measured cross section of about 0.093 pb, yet Eq. (1) and the Conclusion quote sigma(tZq) = 0.81 +/- 0.07 (stat) +/- 0.06 (syst) pb and 0.81 +/- 0.10 pb. The printed value is larger by a factor of about 8.7, and no systematic uncertainty can absorb this discrepancy. The manuscript must either correct a missing-decimal typo (0.081 pb) or re-evaluate the result; as printed, the central numerical claim is not usable.
- [Section 2] The nonprompt-lepton background is a major background in the three-lepton final state, and its estimate relies on a transfer-factor method. The text describes the method only in a qualitative way and reports no transfer-factor uncertainty, closure test, or validation numbers; the validation-region sentence is also internally contradictory ("outside the Z resonance region" followed by |m(ll)-m(Z)| < 20 GeV). Since both measured cross sections shift with this background, the proceedings should either quantify the method or state explicitly that these details and the associated uncertainties are provided only in the companion paper [1].
- [Section 2 (DNN) and Section 3] The claim that the simultaneous measurement is "less dependent on the signal modeling assumptions" is not supported by the material shown. The DNN is trained on the same simulated signal and background models that are used in the fit, and the paper does not report the DNN input variables, training/validation performance, or a comparison of the fitted signal yields with and without the multiclass classification. Without such checks, the reduced model dependence remains an assertion rather than a demonstrated property.
minor comments (5)
- [Section 3] The SM prediction text reads "840 +/- 100 pb" for ttZ; this should presumably be "840 +/- 100 fb", otherwise the sum with tWZ cannot be consistent with the measured 1.14 pb.
- [Section 2] The sentence "events outside the Z boson resonance region are selected, |m(ll)-m(Z)| < 20 GeV" is self-contradictory; the intended selection is likely a sideband with |m(ll)-m(Z)| > 20 GeV.
- [Figure 2] The CMS Preliminary label in Figure 2 reads "1138 fb" while the text states 138 fb^-1; this is a typo and should be corrected.
- [Figure 1] The two-dimensional likelihood plot has no contour levels, color scale, or axis labels visible in the text; this makes the 2D scan difficult to interpret.
- [References] The reference formatting is inconsistent, particularly for reference [3], and DOI/URL formatting should be harmonized with the journal style.
Circularity Check
No circularity in the derivation chain: the cross sections come from a profile-likelihood fit to data, and quoted SM predictions from prior CMS papers are used only as normalization inputs; the apparent tZq unit inconsistency is a numerical/correctness issue, not a circular step.
full rationale
The measurement chain is a standard data-driven extraction. Events are selected and classified by a DNN, and the two cross-section parameters of interest are obtained by a profile-likelihood scan over observed categories. The fitted quantities are cross-section ratios to SM (1.17 +/- 0.07 and 0.99 +/- 0.13); the absolute cross sections in Eq. (1) are obtained by multiplying these fitted ratios by generator-level SM cross sections taken from earlier CMS publications and applying a transfer factor for the mll phase space. Nothing in this chain defines the fitted parameter in terms of the measured output or vice versa; the DNN training on simulated signal templates and the use of the same simulations for signal acceptance are not a logical reduction of the result to its inputs. The self-citations [4]-[6] provide SM expected cross sections and the nonprompt background method, but those are external calculations/measurements, not an unverified theorem invoked to force the result. The paper does contain a serious internal inconsistency: the tZq best-fit ratio 0.99 +/- 0.13 times the quoted SM expectation 94.2 +/- 3.1 fb (with the stated transfer factor) gives about 0.08 pb, not the printed 0.81 +/- 0.10 pb. That is a correctness or typographical concern and should be checked, but it is not a circularity pattern in the enumerated sense. No equation reduces to itself by construction and no fitted input is renamed a prediction, so the circularity score is 0.
Assumptions & free parameters
free parameters (2)
- tZq phase-space transfer factor =
not quoted in the proceedings
- Lepton misidentification rate =
not quoted in the proceedings
assumptions (4)
- domain assumption MadGraph_aMC@NLO v2.6.5 SM predictions are accurate within the quoted theory uncertainties.
- domain assumption The lepton misidentification rate measured in a background-enriched region is valid in the three-lepton signal region.
- domain assumption The multiclass DNN trained on simulated events generalizes to collision data and separates tZq from ttZ+tWZ sufficiently well for the fit.
- standard math Profile likelihood ratio scan provides unbiased confidence intervals for the cross sections.
Cite this review
Pith. "Pith review of First simultaneous measurement of single and pair production of top quarks in association with a Z boson at the LHC." pith.science (2026). https://pith.science/paper/32WMGQDQ
@misc{pith2026250106070,
author = {Pith},
title = {Pith review of: First simultaneous measurement of single and pair production of top quarks in association with a Z boson at the LHC},
year = {2026},
howpublished = {\url{https://pith.science/paper/32WMGQDQ}},
note = {Machine review of arXiv:2501.06070}
}
abstract
The first simultaneous measurement of single and pair production of top quarks in association with a Z boson ($t$Zq, $t$WZ and $t\bar{t}$Z) is presented, including both inclusive and differential cross sections. A multiclass neural network is used to separate the signal and the backgrounds. Compared to previous studies, the simultaneous measurement is less dependent on the signal modeling assumptions and improves the sensitivity to new physics scenarios, as it enables to constrain possible deviations from the standard model across different processes.
Figures
Reference graph
Works this paper leans on
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[1]
, " * write output.state after.block =
ENTRY address archive author booktitle chapter doi edition editor eid eprint howpublished institution isbn journal key month note number organization pages publisher school series title type url volume year label INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all := #1 'mid.sentence := #2 'af...
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[2]
write newline
" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in capitalize " " * FUNCT...
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[3]
CMS Collaboration , Measurements of inclusive and differential cross sections for top quark production in association with a Z boson in proton-proton collisions at s = 13 TeV (2024), 2410.23475
arXiv 2024
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[4]
CMS Collaboration et al , The CMS experiment at the CERN LHC , Journal of Instrumentation 3(08), S08004 (2008), doi:10.1088/1748-0221/3/08/S08004
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[5]
CMS Collaboration , Development of the CMS detector for the CERN LHC Run 3 , Journal of Instrumentation 19(05), P05064 (2024), doi:10.1088/1748-0221/19/05/P05064
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[6]
CMS Collaboration , Inclusive and differential cross section measurements of single top quark production in association with a Z boson in proton-proton collisions at s = 13 TeV , Journal of High Energy Physics 2022(2) (2022), doi:10.1007/jhep02(2022)107
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[7]
CMS Collaboration , Measurement of top quark pair production in association with a Z boson in proton-proton collisions at s = 13 TeV , Journal of High Energy Physics 2020(3) (2020), doi:10.1007/jhep03(2020)056
- [8]
Show all 9 references
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[9]
CMS Collaboration , Differential cross section measurements for the production of top quark pairs and of additional jets using dilepton events from pp collisions at s = 13 TeV (2024), 2402.08486
2024 arXiv
Reviewed August 10, 2026 · model on record in the stance chip above.
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