Charged-particle production in pp collisions at 13.6 TeV and Pb-Pb collisions at 5.36 TeV with ALICE

Tulika Tripathy (for the ALICE collaboration)

REVIEW 4 major objections 5 minor 14 references

Charged-particle production in pp collisions at 13.6 TeV and Pb-Pb collisions at 5.36 TeV with ALICE

T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read This paper reports the first measurements of the charged-particle pseudorapidity density in pp collisions at 13.6 TeV and in Pb-Pb collisions at 5.36 TeV, from ALICE Run 3 data.

desk verdict New ALICE dN/dη points at Run 3 energies are genuine and useful, but the abstract's 'first measurements' claim collides with the CMS paper cited in the same proceedings; easy to fix, but it should be fixed. read the letter →

arxiv 2506.14989 v1 pith:T5S6QH2E submitted 2025-06-17 nucl-ex hep-ex

Tulika Tripathy (for the ALICE collaboration) This is my paper · ORCID

classification nucl-exhep-ex

keywords charged-particlepseudorapiditydensitydN/detappcollisionsat13.6TeVPb-Pb5.36ALICERun3forwardrapiditycentralityclassesPythia8/Angantyr

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

Protons colliding at $\sqrt{s}=13.6$ TeV and lead nuclei at $\sqrt{s_{\rm NN}}=5.36$ TeV are among the highest collision energies reached at a particle collider. This paper reports the first measurements of the charged-particle pseudorapidity density ${\rm d}N_{\rm ch}/{\rm d}\eta$—the number of charged particles produced per unit of pseudorapidity, which measures how far a particle's direction is from the beam axis—in proton-proton collisions at 13.6 TeV and in lead-lead collisions at 5.36 TeV. The values come from upgraded ALICE detector data recorded in LHC Run 3 and set new reference points for how particle production grows with collision energy. They are compared with model predictions and with earlier measurements scaled to the new energies, checking whether the established energy-scaling trend continues.

What carries the argument

The measurement is carried by the upgraded ALICE apparatus: the Inner Tracking System and GEM-based Time Projection Chamber provide midrapidity global tracks, and the newly installed Muon Forward Tracker supplies forward-rapidity tracks in the range $-3.6<\eta<-2.4$. The Fast Interaction Trigger selects the events and, in Pb-Pb, its amplitude is fitted with a Glauber Monte Carlo model to assign centrality classes. Corrections for detector acceptance and tracking efficiency are applied using Pythia8/Angantyr simulations. The central object is the charged-particle pseudorapidity density, ${\rm d}N_{\rm ch}/{\rm d}\eta$, obtained by counting primary charged particles in defined $\eta$ intervals; the energy-scaling comparison uses the observed $(\sqrt{s_{\rm NN}})^{0.156}$ growth of central multiplicity.

What would settle it

If the same raw events were corrected with an independent efficiency estimate—for example from embedding simulated tracks into real collisions or from a tag-and-probe comparison using the overlapping ITS, TPC, and MFT detectors—and the resulting ${\rm d}N_{\rm ch}/{\rm d}\eta$ differed from the quoted values by more than the reported systematic uncertainties, the correction premise would be falsified.

Watch

Extended reading notes

Core claim

The paper reports the first measurements of ${\rm d}N_{\rm ch}/{\rm d}\eta$ at $\sqrt{s}=13.6$ TeV pp and $\sqrt{s_{\rm NN}}=5.36$ TeV Pb-Pb collisions using ALICE Run 3 data. In pp collisions, the midrapidity ($|\eta|<0.5$) result and the forward-rapidity ($-3.6<\eta<-2.4$) result from the new Muon Forward Tracker agree with Pythia8 Monash and with a QCD-based color-reconnection tune; a gluon-move tune overpredicts midrapidity by about 4%, and EPOS 4 underpredicts midrapidity by about 5% and overpredicts forward rapidity by about 15%. In Pb-Pb collisions, the charged-particle density normalized by the mean number of participant nucleons decreases by a factor of about 1.8 from central (0-5%) to peripheral (70-80%) events. Scaled measurements from 2.76 and 5.02 TeV agree with the new 5.36 TeV points, and the ALICE and CMS results agree within uncertainties despite different centrality and pseudorapidity definitions.

Load-bearing premise

The measurement stands on the assumption that the Pythia8/Angantyr simulation reproduces the detector's acceptance and tracking efficiency well enough that the corrections applied to the raw track counts do not bias the reported ${\rm d}N_{\rm ch}/{\rm d}\eta$ values.

Editorial extensions

If this is right

The 13.6 TeV pp measurement gives model builders a new reference point for tuning soft quantum-chromodynamics processes at the highest proton-proton energy at the LHC.
The forward-rapidity points from the Muon Forward Tracker narrow the allowed pseudorapidity-shape predictions, since EPOS 4 overshoots those points by about 15%.
The Pb-Pb result shows that the per-participant charged-particle density falls by roughly a factor 1.8 from central to peripheral collisions at 5.36 TeV, extending the known centrality trend to a new energy.
Multiplying the 2.76 and 5.02 TeV measurements by factors of 1.27 and 1.06 brings them into agreement with the 5.36 TeV data, supporting the same power-law energy growth in central collisions.
Agreement between ALICE and CMS at 5.36 TeV indicates the new reference point is robust against different centrality estimators and different pseudorapidity acceptances.

Reading between the lines

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

A possible extension is to measure dN/deta in pp collisions as a function of event multiplicity, which would show whether the forward-to-midrapidity ratio rises, falls, or stays flat with activity; the current data only cover the inclusive INEL>0 class.
If the forward-rapidity overprediction by EPOS 4 persists after the final published analysis, the MFT's forward coverage would become a sharper testing ground for hydrodynamical versus non-hydrodynamical treatment of the event periphery.
The scaling comparison uses fully uncorrelated uncertainties between data sets; a joint analysis treating the common systematic sources would let the quoted 0.156 exponent be tested more stringently at 5.36 TeV.

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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 read of the ALICE proceedings. What is actually new: dN_ch/dη values in pp at 13.6 TeV (mid- and forward-rapidity via MFT) and Pb–Pb at 5.36 TeV across centralities. Those are real data points, not in the earlier ALICE papers. The forward-rapidity pp measurement is a first for ALICE, and the midrapidity pp point extends the energy series. The analysis follows the standard ALICE recipe: INEL>0 selection, global plus ITS-only tracks, Pythia8/Angantyr corrections, Glauber centrality from FT0. That is sound and the cross-checks against scaled 2.76 and 5.02 TeV data and against CMS look consistent. Credit where due: the paper is compact, the figures are informative, and the model comparisons (Pythia8 tunes, EPOS4) give the reader something to think about rather than just numbers.

The soft spot that matters is the abstract. It says these are the first measurements of dN_ch/dη in Pb–Pb at 5.36 TeV. Then Section 4.2 and Figure 2 compare with CMS PLB 861 (2025) 139279 at the same energy, calling them 'recent results from the CMS experiment.' If CMS reported the same observable at that energy—which the comparison strongly implies—then the ALICE result is not first; it is first by ALICE, or first with these acceptance cuts. The stress-test note lands on this. It is the central claim of the abstract, and it is wrong as written. The fix is easy: say 'first ALICE measurements' or 'first ALICE results at these energies' and let the data speak. As is, a referee would rightly bounce it.

Other soft spots are more minor. There are no numerical tables of the dN/dη values or the systematic breakdown; that is normal for a proceedings, but it means the reader cannot independently check the quoted uncertainties. The rate of typos is a bit high ('0-5$\%)', 'T eV', 'Pb-–Pb'), which is cosmetic. The use of the 0.156 scaling exponent from the prior ALICE paper is fine—it is used only to place old data on the same energy scale for comparison, and it does not constrain the new measurement. The Pythia8/Angantyr efficiency correction is the usual load-bearing assumption; the backup is that the same method matched CMS at 5.36 TeV, which is reassuring.

Who benefits from this paper: people building or constraining QCD Monte Carlo generators, and the heavy-ion soft-QCD community. It is a conference proceedings, so it will not be a landmark, but it contains the first numbers at these energies from ALICE and deserves to be citable. I would send it to peer review at whatever venue collects these proceedings, provided the first-measurements claim is corrected. With that correction, it is a legitimate, incremental but useful contribution.

Referee Report

4 major / 5 minor

Summary. This proceedings paper reports ALICE measurements of the charged-particle pseudorapidity density, dN_ch/dη, in pp collisions at √s = 13.6 TeV (midrapidity |η|<0.5 and forward rapidity −3.6<η<−2.4 using the new MFT) and in Pb–Pb collisions at √s_NN = 5.36 TeV for centrality classes 0–5% to 70–80%. The results are compared with Pythia8 tunes and EPOS4, with a CMS Pb–Pb measurement at the same energy, and with ALICE data at lower energies scaled by a power law. The paper is a conference proceeding: it describes the event selection, track types, and Monte-Carlo-based corrections, but contains no numeric tables, no detailed systematic breakdown, and no public data release.

Significance. If the numerical results are correct, the pp forward-rapidity dN/dη measurement with the MFT is a genuinely new ALICE result, and the Pb–Pb data provide a useful cross-check of particle production at a new LHC energy. The large event samples (3.0/2.1/1.1 billion events for the three analyses) and the comparisons with the CMS result and with energy-scaled ALICE data give the paper value as a preliminary reference. However, the central novelty claim as written is not supported for the Pb–Pb leg, and the scaling comparison contains a numerical inconsistency. These issues are correctable, and the underlying measurement appears plausible, but the current version needs revision before the claims can be accepted.

major comments (4)

[Abstract and Section 4.2] The abstract claims 'first measurements' of dN_ch/dη in pp at 13.6 TeV and in Pb–Pb at 5.36 TeV. Section 4.2 and Figure 2 compare the new ALICE Pb–Pb results with CMS (PLB 861 (2025) 139279) at the same √s_NN = 5.36 TeV. That CMS paper reports charged-particle pseudorapidity densities in Pb–Pb at this energy, as the comparison in Figure 2 confirms. The claim should be restricted to 'first ALICE measurements' or otherwise qualified (e.g., 'first combined midrapidity and forward-rapidity measurements'); as written, the central novelty claim is contradicted by the manuscript's own cited reference.
[Section 4.2] The text states that the scaling factors 1.27 (for 2.76 TeV) and 1.06 (for 5.02 TeV) 'correspond to the observed √s_NN^0.156 dependence of multiplicity in central collisions'. Direct calculation gives (5.36/2.76)^0.156 = 1.109 and (5.36/5.02)^0.156 = 1.010. The stated factors are therefore inconsistent with the quoted exponent. Please clarify how the 1.27 and 1.06 factors are derived, or correct the exponent; the claim of good agreement after scaling depends on this.
[Section 3] The corrections for acceptance and tracking efficiency are applied using Pythia8/Angantyr simulations, but no systematic uncertainty associated with this correction is given, and no validation (closure tests, comparison of simulated and measured track distributions) is presented. The reported dN/dη values and the comparison with models in Figures 1–2 depend directly on these corrections, so the systematic budget is incomplete as presented.
[General] No numeric results are tabulated: neither the dN_ch/dη values nor the centrality-dependent values in Figure 2 are given with their statistical and systematic uncertainties; the figures only show bands. As a measurement paper, this prevents the reader from using the results or checking the quoted uncertainties. A table of values or a public data release should be added, or at least the results should be made available as supplemental material.

minor comments (5)

[Abstract and Introduction] The phrase '0-5%)' contains an unmatched parenthesis; it should read '0–5%'.
[Abstract, Sections 1 and 4.1] The forward-rapidity range is written as '−3.6<|η|<−2.4', which is mathematically impossible because |η| is non-negative. It should be '−3.6<η<−2.4' or '2.4<|η|<3.6'.
[Section 4.1] The sentence 'The ALICE experiment is able to measure charged-particle multiplicity density with tracks in the central and forward rapidity region for the first time' is ambiguous; specify whether this refers to the first simultaneous measurement in both regions with ALICE.
[Section 3] The version of Pythia8/Angantyr used is not specified; please add the version number.
[Section 4.2] The phrase 'assuming full uncorrelation between data sets' should be rephrased, e.g., 'assuming fully uncorrelated systematic uncertainties between data sets.'

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the ALICE measurement is self-contained; the only self-cited input is a prior energy-scaling exponent used for comparison scaling, not for deriving the new dN/deta values.

full rationale

This is a measurement paper, not a derivation. The central quantities (dN_ch/deta in pp and Pb-Pb) are obtained by counting primary charged particles and applying acceptance/efficiency corrections from Pythia8/Angantyr simulations; no parameter of the new measurement is fitted to the same observable, and the model comparisons are external benchmarks rather than inputs to the corrected values. The only self-citation is Ref. [1], used to obtain the sqrt(s_NN)^0.156 scaling factors for comparing previous ALICE data at 2.76 and 5.02 TeV with the new 5.36 TeV points. That is a comparison convention, not an input to the new measurement, and it does not constrain the reported dN/deta values. The abstract's 'first measurements' wording is questionable in light of the cited CMS PLB 861 (2025) 139279 results at the same Pb-Pb energy, but priority/factual consistency is not a circularity pattern. No equation in the paper reduces to an input, no fitted parameter is renamed as a prediction, and no load-bearing claim rests solely on a self-citation chain.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

This is a measurement paper with no new theoretical entities. The central numbers rest on standard detector-response simulations and a prior scaling law, both disclosed. The only numerical constant imported from elsewhere is the 0.156 scaling exponent. No parameters are fit to the new data in this paper.

free parameters (1)

Energy scaling exponent for Pb-Pb multiplicity = 0.156
Taken from prior ALICE result [1] and used to scale 2.76 and 5.02 TeV data by 1.27 and 1.06 for comparison with 5.36 TeV. Not fitted in this paper.

assumptions (4)

domain assumption Pythia8/Angantyr simulations accurately model detector acceptance and tracking efficiency for the correction of raw counts.
Section 3: 'Corrections for acceptance and tracking efficiency are applied using Pythia8/Angantyr simulations.'
domain assumption The FT0 amplitude fitted with a Glauber Monte Carlo model provides an unbiased estimate of collision centrality in Pb-Pb.
Section 3: 'Centrality in Pb-Pb collisions is determined by fitting the FT0 amplitude using a Glauber MC model.'
domain assumption The charged-particle multiplicity in central Pb-Pb scales as s_NN^0.156, justifying the scaling of older data.
Section 4.2: scaling factors 1.27 and 1.06 'correspond to the observed s_NN^0.156 dependence of multiplicity in central collisions'.
domain assumption INEL>0 event selection with vertex cuts defines a consistent inelastic event sample for both collision systems.
Section 3: 'Inelastic events with at least one charged particle within |η|<1 (INEL>0 event class) are used.'

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Cite this review

Pith. "Pith review of Charged-particle production in pp collisions at 13.6 TeV and Pb-Pb collisions at 5.36 TeV with ALICE." pith.science (2026). https://pith.science/paper/T5S6QH2E

@misc{pith2026250614989,
  author       = {Pith},
  title        = {Pith review of: Charged-particle production in pp collisions at 13.6 TeV and Pb-Pb collisions at 5.36 TeV with ALICE},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/T5S6QH2E}},
  note         = {Machine review of arXiv:2506.14989}
}

abstract

The first measurements of the charged-particle pseudorapidity density, ${\rm d}N_{\rm{ch}}/{\rm d}\eta$, in proton-proton (pp) collisions at $\sqrt{s} =$ 13.6 TeV and in lead-lead (Pb-Pb) collisions at $\sqrt{s_{\rm{NN}}} = $ 5.36 TeV are presented. The analysis is based on Run 3 data recorded in 2022 and 2023, using the upgraded ALICE detector at the LHC. The average charged-particle pseudorapidity density $\langle{\rm d}N_{\rm{ch}}/{\rm d}\eta\rangle$ in pp collisions was measured at midrapidity ($|\eta|<$0.5) with the upgraded central barrel detectors, such as the Inner Tracking System and the Time Projection Chamber and, at forward pseudorapidity ($-$3.6 $<|\eta|<-$ 2.4), using the newly installed Muon Forward Tracker (MFT). For Pb-Pb collisions, the measurement was performed in different centrality classes, ranging from the most central 0-5$\%$) to the peripheral (70-80$\%$) collisions.

Discussion (0). Continue with ORCID to comment.

Reference graph

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

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Acharya et al

S. Acharya et al. [ALICE], [arXiv:2504.02505 [nucl-ex]]

arXiv

[2] [2]

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work page 2024

[3] [3]

Loizides, J

C. Loizides, J. Kamin and D. d'Enterria, Phys. Rev. C 97, no.5, 054910 (2018)

work page 2018

[4] [4]

Skands, S

P. Skands, S. Carrazza and J. Rojo, Eur. Phys. J. C 74, no.8, 3024 (2014)

work page 2014

[5] [5]

J. R. Christiansen and P. Z. Skands, JHEP 08, 003 (2015)

2015

[6] [6]

Bierlich, S

C. Bierlich, S. Chakraborty, N. Desai, L. Gellersen, I. Helenius, P. Ilten, L. L\"onnblad, S. Mrenna, S. Prestel and C. T. Preuss, et al. SciPost Phys. Codeb. 2022, 8 (2022)

work page 2022

[7] [7]

Hayrapetyan et al

A. Hayrapetyan et al. [CMS], Phys. Lett. B 861, 139279 (2025)

work page 2025

[8] [8]

Aamodt et al

K. Aamodt et al. [ALICE], Phys. Rev. Lett. 106, 032301 (2011)

work page 2011