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arxiv: 2604.27091 · v1 · submitted 2026-04-29 · ⚛️ nucl-ex · hep-ex

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Measurement of the top quark pair production cross section in PbPb collisions at sqrt{s_NN} = 5.36 TeV

CMS Collaboration
Authors on Pith no claims yet

Pith reviewed 2026-05-07 09:52 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-ex
keywords top quark pair productionlead-lead collisionscross section measurementperturbative quantum chromodynamicsnuclear parton distribution functionsdilepton final statesDrell-Yan processcentrality dependence
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The pith

The top quark pair production cross section in lead-lead collisions at 5.36 TeV per nucleon pair is measured to be 3.42 micro barns.

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

This paper reports the first measurement of the inclusive cross section for top quark pair production in lead-lead collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. The analysis uses dilepton final states and extracts the signal via a fit to a multivariate discriminator combining lepton kinematics and bottom quark jet multiplicity. The measured value is consistent with next-to-next-to-leading order perturbative quantum chromodynamics calculations that incorporate nuclear parton distribution functions. Separate results are given for the Drell-Yan cross section, their ratio, and for central versus semicentral collisions to examine dependence on collision geometry.

Core claim

The central claim is that the inclusive top quark pair production cross section in lead-lead collisions at 5.36 TeV per nucleon pair equals 3.42^{+0.54}_{-0.51}(stat)^{+0.50}_{-0.43}(syst) micro barns. This is extracted from dilepton events using a fit to a multivariate discriminator and agrees with NNLO pQCD predictions employing several nuclear PDFs. The Drell-Yan cross section for dilepton masses above 10 GeV and the ratio of top pair to Drell-Yan cross sections are also compatible with the same calculations. The observables are measured separately in central and semicentral collisions to study impact parameter dependence for the first time.

What carries the argument

A multivariate discriminator that combines the kinematic properties of decay electrons and muons with the multiplicity of bottom quark jets, used inside a fit to isolate the top quark pair signal after background subtraction.

If this is right

  • The result tests the applicability of perturbative quantum chromodynamics calculations to heavy quark production in nuclear collisions.
  • Separate measurements in central and semicentral collisions provide the first experimental handle on the impact parameter dependence of top quark production.
  • The ratio to the Drell-Yan cross section supplies an additional observable for constraining nuclear modifications to parton distributions.
  • Consistency with theory supports the use of these calculations for modeling other rare processes in nuclear collisions.
  • The observables can serve as a baseline for future studies of top quark behavior in dense nuclear matter.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Top quarks, which decay before the formation of a quark-gluon plasma, could serve as probes of the initial state in heavy-ion collisions if the measurement is repeated at higher luminosities.
  • The analysis technique may be extended to other rare processes to test for deviations from standard model expectations in nuclear environments.
  • Agreement with nuclear PDFs suggests that top quark data can help reduce uncertainties on gluon distributions inside nuclei at high momentum fractions.
  • If the lack of strong centrality dependence persists, it would imply that top production is largely insensitive to the density of the colliding system.

Load-bearing premise

The measurement assumes that the multivariate discriminator correctly isolates the top quark pair signal after all background subtractions and efficiency corrections, and that the selected nuclear parton distribution functions accurately represent the initial-state nuclear modifications.

What would settle it

A significantly different cross section value extracted from an independent dataset with higher integrated luminosity or at a different collision energy would indicate either incomplete background modeling or inaccuracies in the nuclear parton distribution functions.

Figures

Figures reproduced from arXiv: 2604.27091 by CMS Collaboration.

Figure 1
Figure 1. Figure 1: Distributions of dilepton pT (ℓℓ) (left) and invariant mass m(ℓℓ) (right) variables in the same-flavor (ℓℓ) channels. The m(ℓℓ) distribution is shown before applying the veto of the Z boson resonant region. The data (black markers with error bars representing statistical uncertainties) are overlaid on top of the stacked contributions from the expected tt (blue), non￾prompt (light orange), WW (red), single … view at source ↗
Figure 2
Figure 2. Figure 2: Distributions of pT (ℓ2 ) (upper left), pT (ℓℓ) (upper right), and ∑|ηℓ | (lower) variables in the different-flavor (e∓µ ±) channel. The data (black markers with error bars representing statistical uncertainties) are overlaid on top of the stacked contributions from the expected tt (blue), nonprompt (light orange), WW (red), single top quark (gray), and DY (violet) processes. The last bin includes the over… view at source ↗
Figure 3
Figure 3. Figure 3: Distribution of the xj = pT (jet)/pT (Z) ratio in Z + jet events in the 0–10% (left) and 10–90% (right) PbPb centralities. The data (solid dots with error bars) are compared with the contributions from UE jets (gray histograms) and true recoil jets (black dashed curves) stacked. The blue curves show the sum of UE and recoil jet contributions. Among all the parameters of the fit, the mean of the Crystal-Bal… view at source ↗
Figure 4
Figure 4. Figure 4: Ratio of the average ⟨xj ⟩ detector-level distributions measured in data over simula￾tion as a function of the collision centrality with a fit to an error function (blue dashed curve) superimposed. clusive and/or bottom-quark jets. We therefore validate the approach by fitting the jet and b jet multiplicities in Z boson events. The results of these fits are displayed in view at source ↗
Figure 5
Figure 5. Figure 5: Multiplicities of inclusive (upper) and b-tagged (lower) jets in Z boson candidate view at source ↗
Figure 6
Figure 6. Figure 6: Distributions of the final BDT discriminator after the fit for the same-flavor view at source ↗
Figure 7
Figure 7. Figure 7: Scan of the profile likelihood as a function of the t view at source ↗
Figure 8
Figure 8. Figure 8: Impact of systematic uncertainties on the fitted t view at source ↗
Figure 9
Figure 9. Figure 9: Experimental measurements (colored bands) of the t view at source ↗
Figure 10
Figure 10. Figure 10: Experimental measurements (colored bands) of the t view at source ↗
read the original abstract

The inclusive cross section for top quark pair ($\mathrm{t\bar{t}}$) production in lead-lead (PbPb) collisions is reported for the first time at a center-of-mass energy per nucleon pair of 5.36 TeV. The analysis uses data corresponding to an integrated luminosity of 1.58 nb$^{-1}$ collected by the CMS experiment at the CERN LHC in 2023. The $\mathrm{t\bar{t}}$ production cross section, $\sigma_{\mathrm{t\bar{t}}} $ = 3.42$^{+0.54}_{-0.51}$(stat)$^{+0.50}_{-0.43}$(syst) $\mu$b, is measured in dilepton final states using a fit to a multivariate discriminator that combines the decay electron and muon kinematic properties with the multiplicity of bottom quark jets. The result is consistent with perturbative quantum chromodynamics calculations at next-to-next-to-leading order (NNLO) accuracy employing several nuclear parton distribution functions. In addition, the Drell$-$Yan production cross section ($\sigma_\text{DY}$) for dilepton masses above 10 GeV and the ratio of $\mathrm{t\bar{t}}$ to DY cross sections ($R_{\mathrm{t\bar{t}}/\mathrm{DY}}$) are found to be compatible with the NNLO predictions. The observables $\sigma_{\mathrm{t\bar{t}}}$, $\sigma_\text{DY}$, and $R_{\mathrm{t\bar{t}}/\mathrm{DY}}$ are measured separately for central and semicentral PbPb collisions to investigate for the first time the dependence of top quark production on the collision impact parameter.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports the first measurement of the inclusive top-quark pair production cross section in PbPb collisions at √s_NN = 5.36 TeV. Using 1.58 nb^{-1} of CMS data in dilepton final states, the cross section σ_ttbar = 3.42^{+0.54}_{-0.51}(stat)^{+0.50}_{-0.43}(syst) μb is extracted via a fit to a multivariate discriminator that combines lepton kinematics and b-jet multiplicity. The result is stated to be consistent with NNLO pQCD calculations employing several nuclear PDFs. Additional measurements of the Drell-Yan cross section above 10 GeV and the ttbar/DY ratio are reported, including separate results for central and semicentral collisions to probe impact-parameter dependence.

Significance. If the background modeling and efficiency corrections are robust, this constitutes the first experimental benchmark of top-quark production in heavy-ion collisions at LHC energies. It tests the applicability of pQCD in a nuclear environment, provides a potential handle on nuclear PDFs, and introduces centrality-dependent observables that could reveal medium or initial-state effects. The data-driven fit approach and post-extraction theory comparison are positive features.

major comments (2)
  1. [Analysis and fit procedure] The extraction of σ_ttbar rests on the multivariate discriminator fit after background subtraction and efficiency corrections. The analysis must demonstrate that the simulation accurately reproduces the discriminator shape (lepton kinematics + b-jet multiplicity) for both ttbar signal and dominant DY background in the high-multiplicity PbPb underlying event; any mismatch in b-tagging efficiency or jet multiplicity due to nuclear effects would directly bias the fitted yield. This assumption is load-bearing for the central claim.
  2. [Results] The reported result separates statistical and systematic uncertainties, but the breakdown of the dominant systematic sources (e.g., b-tagging, lepton isolation, jet energy scale, and pileup modeling in PbPb) is not detailed in the provided description. A table quantifying each contribution is needed to evaluate whether the total syst uncertainty of +0.50/-0.43 μb is realistic.
minor comments (2)
  1. [Results] The abstract states consistency with NNLO predictions using 'several' nuclear PDFs; the main text should explicitly list the PDF sets employed and show the theory uncertainty bands for a quantitative comparison.
  2. [Introduction] Notation for the cross-section uncertainties is clear, but the integrated luminosity (1.58 nb^{-1}) should be accompanied by its uncertainty when first introduced.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review of our manuscript and for the positive evaluation of its significance. We address the major comments point by point below, making revisions to the manuscript where appropriate to improve clarity and robustness.

read point-by-point responses

  1. Referee: [Analysis and fit procedure] The extraction of σ_ttbar rests on the multivariate discriminator fit after background subtraction and efficiency corrections. The analysis must demonstrate that the simulation accurately reproduces the discriminator shape (lepton kinematics + b-jet multiplicity) for both ttbar signal and dominant DY background in the high-multiplicity PbPb underlying event; any mismatch in b-tagging efficiency or jet multiplicity due to nuclear effects would directly bias the fitted yield. This assumption is load-bearing for the central claim.

    Authors: We agree that validating the simulation's reproduction of the discriminator shape is critical for the reliability of the extracted cross section. The manuscript already presents comparisons of the individual input variables to the discriminator (lepton kinematics and b-jet multiplicity) in data and simulation for various control samples in Section 6. To directly address the referee's concern, we have added new figures in the revised manuscript showing the full multivariate discriminator distribution in a background-dominated control region and in the signal region, with data and simulation overlaid. These show good agreement. We have also quantified the impact of potential nuclear effects on b-tagging by performing a data-driven calibration in PbPb collisions and included an additional uncertainty on the b-tagging efficiency to cover any residual discrepancies. This ensures the fit is robust against the mentioned biases. revision: yes

  2. Referee: [Results] The reported result separates statistical and systematic uncertainties, but the breakdown of the dominant systematic sources (e.g., b-tagging, lepton isolation, jet energy scale, and pileup modeling in PbPb) is not detailed in the provided description. A table quantifying each contribution is needed to evaluate whether the total syst uncertainty of +0.50/-0.43 μb is realistic.

    Authors: We appreciate this comment and agree that a detailed breakdown enhances the transparency of the result. Although the main sources are discussed in the text of the original manuscript, we have now included a comprehensive table in the revised version that breaks down the systematic uncertainty contributions from b-tagging efficiency, lepton isolation and identification, jet energy scale, pileup modeling in PbPb, and other sources. These are combined in quadrature to yield the total systematic uncertainty. This addition allows readers to assess the realism of the uncertainty estimate. revision: yes

Circularity Check

0 steps flagged

No circularity: cross section extracted directly from data via discriminator fit; theory used only for post-hoc comparison

full rationale

The paper performs an experimental measurement of the ttbar cross section in PbPb collisions by fitting a multivariate discriminator (lepton kinematics + b-jet multiplicity) to collision data after background subtraction and efficiency corrections. The extracted yield determines σ_ttbar independently of any theoretical input. NNLO pQCD calculations with nuclear PDFs enter only after the measurement for consistency checks and do not participate in the fit or corrections. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations appear in the derivation chain. The result is self-contained against the recorded data and is falsifiable by independent experiments or alternative analysis choices.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The result rests on standard collider-analysis assumptions (background modeling, detector response simulation, and nuclear PDF inputs) drawn from prior literature; no new free parameters, axioms, or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption Standard assumptions in particle physics event selection, background estimation, and efficiency corrections hold.
    Typical for LHC heavy-ion analyses; invoked implicitly by the reported measurement procedure.

pith-pipeline@v0.9.0 · 5603 in / 1283 out tokens · 64239 ms · 2026-05-07T09:52:45.391755+00:00 · methodology

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