Anisotropic Flow in Ultra-Central Pb$-$Pb Collisions at $\sqrt{\mathrm{s_{NN}}}=5.36$ TeV with ALICE

Iris Likmeta

REVIEW 4 major objections 4 minor 14 references

Anisotropic Flow in Ultra-Central Pb$-$Pb Collisions at $\sqrt{\mathrm{s_{NN}}}=5.36$ TeV with ALICE

T0 review · 4 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read ALICE finds that triangular-to-elliptic flow in ultra-central Pb-Pb at 5.36 TeV exceeds hydrodynamic model predictions.

desk verdict Useful ALICE Run 3 anchor for the ultra-central flow puzzle, but the headline discrepancy with Trajectum rests on systematics that the paper doesn't yet document. read the letter →

arxiv 2608.11199 v1 pith:MM4GM75G submitted 2026-08-11 nucl-ex hep-ex

Iris Likmeta This is my paper · ORCID

classification nucl-exhep-ex

keywords anisotropicflowultra-centralcollisionstriangularellipticquark-gluonplasmahydrodynamicmodelnucleardeformationBessel-Gaussianlimit

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 paper reports ALICE Run 3 measurements of the ratio of triangular to elliptic flow, $v_3\{2\}/v_2\{2\}$, in central and ultra-central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV, down to the top 0.005% of the cross section. The data agree with the earlier Run 2 measurement and with the Trajectum hydrodynamic model in the 0-5% range, but in the ultra-central bins the measured ratio rises above the model prediction. The paper presents this as experimental evidence that the current initial-state and hydrodynamic description of the most central collisions is incomplete. It also reports the first Run 3 measurement of $v_3\{4\}/v_3\{2\}$ in 0-5%, which is consistent with the Bessel-Gaussian limit and supports a fluctuation-dominated origin for triangular flow. The working explanation offered is an intrinsic octupole deformation of the $^{208}$Pb nucleus.

What carries the argument

The argument runs on two-particle and four-particle azimuthal cumulants measured within the generic framework, with a pseudorapidity gap $|\Delta\eta|>1$ to suppress non-flow. The ratio $v_3\{2\}/v_2\{2\}$ isolates the fluctuation-driven triangular response from the geometry-driven elliptic response as the average overlap becomes round, while $v_3\{4\}/v_3\{2\}$ tests whether the four-particle cumulant is consistent with the Bessel-Gaussian picture of random initial-state fluctuations. The ultra-central centrality bins, down to 0-0.005%, are the regime where these ratios are most discriminating; the comparison with the hydrodynamic model, which assumes a linear response $v_n\propto\varepsilon_n$, turns the measured ratio into a test of the initial-state parametrization. Run 3 continuous readout forces an occupancy cut and non-uniform acceptance and efficiency (NUA/NUE) corrections, which the analysis uses to control the high-rate environment.

What would settle it

A closure test comparing reconstructed and generator-level $v_3/v_2$ in the LHC24g3 Monte Carlo sample at ultra-central multiplicities would reveal whether occupancy-dependent efficiency corrections bias the ratio.

Watch

Extended reading notes

Core claim

The paper's central claim is that the ratio $v_3\{2\}/v_2\{2\}$ in ultra-central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV is not reproduced by the Trajectum hydrodynamic model once the centrality drops below about 1%. In less extreme centralities the new data agree with both the previous ALICE Run 2 results and the model, so the discrepancy is specific to the regime where the average overlap geometry is nearly round and fluctuations carry the flow. The measurement of $v_3\{4\}/v_3\{2\}$ in 0-5% centrality, equal to the Bessel-Gaussian expectation $\sigma_{v_3}/\langle v_3\rangle\approx\sqrt{4/\pi-1}\approx0.52$, indicates that triangular flow in that range is driven by event-by-event initial-state fluctuations rather than by a coherent geometric source. Together the two results imply that the missing triangular flow in ultra-central collisions must come from a fluctuation source not present in current initial-state models, with an octupole deformation of $^{208}$Pb as the proposed candidate.

Load-bearing premise

The analysis assumes the occupancy cut and NUA/NUE corrections leave the ultra-central $v_3/v_2$ ratio unbiased, yet it provides no closure test or independent track cross-check to verify this.

Editorial extensions

If this is right

If the ultra-central discrepancy is physical, current initial-state models must add a fluctuation source beyond nucleon positions, and an octupole deformation $\beta_3$ of $^{208}$Pb is a concrete candidate that can be implemented and tested.
The crossing where normalized $v_3\{2\}$ equals $v_2\{2\}$ as a function of charged-particle multiplicity marks the centrality at which fluctuation-driven triangular flow overtakes average-geometry elliptic flow; any successful model must reproduce this crossing.
The $v_3\{4\}/v_3\{2\}$ result establishes a fluctuation-dominated baseline in 0-5% centrality, so future ultra-central measurements of the same ratio can detect the onset of a coherent geometric contribution if one appears.
A hydrodynamic model that includes octupole deformation should simultaneously describe the enhanced $v_3\{2\}/v_2\{2\}$ ratio and the $v_3\{4\}/v_3\{2\}$ baseline, providing a sharper test than the ratio alone.

Reading between the lines

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

A natural extension is to treat the shear viscosity inferred from Bayesian fits as potentially biased: if the initial-state model lacks deformation degrees of freedom, part of the flow signal now attributed to viscosity could instead be absorbed by nuclear shape parameters.
The same measurement with xenon or uranium beams would separate deformation-driven effects from universal fluctuation physics, since those nuclei have different ground-state deformations than $^{208}$Pb.
If future ultra-central data show $v_3\{4\}/v_3\{2\}$ staying at the Bessel-Gaussian value while $v_3\{2\}/v_2\{2\}$ continues to exceed models, the excess triangular flow would have to be fluctuation-driven rather than coherent, pointing to deformed nucleon distributions rather than a global octupole shape.
A fast experimental check would be to vary the occupancy cut in the 0-0.2% centrality bins and watch whether the reported ratio shifts; the paper shows no such closure test, so this is the quickest way to test robustness.

Share X Bluesky LinkedIn Reddit HN

Signed reviews

No signed human review yet.

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

You should know this is a short ALICE proceedings contribution that gives the first Run 3 look at v3{2}/v2{2} in ultra-central Pb-Pb at 5.36 TeV, plus a new v3{4}/v3{2} measurement in 0-5%. The physics: agreement with Run 2 and Trajectum down to 1% centrality, then a clear divergence inside the top 1%, consistent with the known ultra-central flow puzzle. That is a useful new data anchor, but the paper is not the final word.

The measurement is a standard two- and multi-particle cumulant analysis, competently executed within an established program. The v3{4}/v3{2} result sitting on the Bessel-Gaussian limit is a nice confirmation that triangular flow in 0-5% is fluctuation-dominated. The paper is appropriately modest in tone: it presents a discrepancy without claiming to resolve the puzzle. Credit where due: it ships no code or data tables, but for a proceedings it is a clear, honest write-up.

Now the soft spots, and they are real. The central claim is the discrepancy with Trajectum in the most central bins, and the paper gives no numerical values, no systematic budget for those bins, no closure test, and no cross-check with a second centrality estimator. Run 3 operates at high occupancy; an occupancy cut and NUA/NUE corrections are mentioned but not validated in the ultra-central region. Since v2{2} is small and steeply centrality-dependent there, a small occupancy-driven migration or track-efficiency modulation could move v3/v2 by more than the quoted uncertainty. The agreement with Run 2 is stated only up to 1% centrality, so it does not cover the bins where the discrepancy appears. The Trajectum comparison is also taken as a baseline without quantifying model uncertainty or showing the model's separate v2 and v3 trends; fine for a preview, but the phrase 'clear discrepancy' is stronger than the evidence shown.

The stress-test note about occupancy bias is not a straw man; it lands. But for a conference proceedings this is a missing validation rather than a disqualifying error. The measurement is plausible, the method is standard, and the result is consistent with prior CMS-based tension. The main thing I want is the follow-up with systematics and closure tests, which the paper explicitly promises.

Who is this for? People working on initial-state modeling, nuclear deformation, and the UCC puzzle. They will want the numbers, and this paper doesn't give them, so it is not citable for quantitative work yet. As a proceedings contribution, it deserves a serious referee; the analysis is not sloppy, just incomplete in the way a preliminary note often is. My recommendation: accept with minor revision, with a clear request that the ultra-central systematic budget and a centrality cross-check be added or explicitly deferred to the upcoming archival paper. If the venue expects full experimental detail, this falls short of that bar.

Referee Report

4 major / 4 minor

Summary. This proceedings-style paper reports preliminary ALICE Run 3 measurements of the triangular-to-elliptic flow ratio, v3{2}/v2{2}, in ultra-central Pb–Pb collisions at sqrt(s_NN)=5.36 TeV. The data are compared with ALICE Run 2 results and with predictions from the Bayesian-tuned Trajectum hydrodynamic model. The paper claims agreement with Run 2 and Trajectum for centralities down to about 1%, but a clear discrepancy in the most central bins (down to 0–0.005%), which is interpreted as evidence that current initial-state/hydrodynamic modeling is incomplete and possibly pointing to octupole deformation of 208Pb. The paper also reports a first Run 3 measurement of v3{4}/v3{2} in 0–5% centrality and finds it consistent with the Bessel–Gaussian limit, supporting a fluctuation-dominated origin of triangular flow.

Significance. If the reported ultra-central discrepancy is robust, the result would sharpen the so-called UCC puzzle by extending it to 5.36 TeV and by constraining models that invoke octupole deformation of 208Pb. The comparison with Trajectum is a useful external baseline, and the v3{4}/v3{2} result provides an independent check on the fluctuation picture. However, the paper is a short contribution with no numerical tables, no systematic-uncertainty decomposition, and no closure or cross-check tests; the central claim is therefore plausible but not yet quantitatively established. The significance is mostly prospective, pending the documented analysis details.

major comments (4)

[Section 3, Fig. 3] The central claim that the v3{2}/v2{2} ratio in ultra-central collisions is no longer reproduced by Trajectum rests entirely on plotted points and a shaded band. No numerical values, no centrality-bin table, and no breakdown of statistical versus systematic uncertainties are given. Without these, the size and significance of the discrepancy cannot be independently assessed. A table (or a reference to a public ALICE preliminary result) with the central values and the full systematic budget for each UCC bin is required.
[Section 2, occupancy and tracking] The manuscript acknowledges that Run 3 operates at high detector occupancy and that an occupancy cut is applied, but it provides no closure test, no variation of the occupancy cut, and no comparison between independent track samples (e.g., global tracks alone versus global+ITS-only tracks). Because v2 is small and steeply centrality-dependent in the ultra-central bins, even a small occupancy-driven acceptance or efficiency modulation can bias v3/v2, which has v2 in the denominator. This is a load-bearing concern for the claimed discrepancy and must be addressed before the ultra-central result can be considered robust.
[Section 3, Figs. 1 and 3] The text states that Fig. 1 shows good agreement with Run 2 and Trajectum up to 1% centrality, while Fig. 3 shows a clear discrepancy in bins that include 0–1% and 0–2.5%. The relationship between the two figures is unclear: does Fig. 1 include the same 0–1% bin as Fig. 3, and if so, how is the agreement in Fig. 1 compatible with the discrepancy in Fig. 3? The centrality definitions and the exact overlap of the plotted intervals need to be clarified, or one of the statements must be qualified.
[Section 3, Fig. 4] The v3{4}/v3{2} measurement is presented as consistent with the Bessel–Gaussian limit, with sigma_v3/<v3> approximately 0.52, but no uncertainties are quoted in the text and no information is given on how nonflow or centrality binning affects the comparison. Since this result is used to support the fluctuation-dominated interpretation, the analysis details and numerical values should be provided so that the consistency claim can be evaluated.

minor comments (4)

[Abstract and Section 1] There are small typographical issues, such as "produced particle' momenta" in the abstract; the possessive should be "particles' momenta." Equation (1) is also poorly typeset with "∞X" and "cos[n(φ−Ψ n)]" appearing inline.
[Section 2] The text should specify the Run 2 comparison energy: Run 2 Pb–Pb data at 5.02 TeV are compared with the new 5.36 TeV results, and the possible implications of the energy difference for the flow ratio should be stated or explicitly neglected.
[Figure captions] The captions for Figs. 3 and 4 state that shaded bands represent systematic uncertainties, but the method used to estimate these systematics is not described anywhere in the text. A brief description or reference to the ALICE analysis note is needed.
[Section 3] The statement that Trajectum assumes a linear hydrodynamic response (vn proportional to epsilon_n) is an oversimplification; although such a relation is often used for response analyses, Trajectum itself performs full hydrodynamic evolution. Consider rephrasing to avoid misrepresenting the model.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the paper is a measurement–model comparison with no fitted inputs renamed as predictions.

full rationale

The paper contains no derivation chain whose output is equivalent to its input. The central claim is that the measured v3{2}/v2{2} ratio in ultra-central Pb–Pb collisions at 5.36 TeV is not reproduced by the Trajectum hydrodynamic model (Fig. 3). This is a direct comparison between an independent ALICE Run 3 measurement and an externally published model calculation; no parameter of the model is fitted to the data presented here, and no quantity is defined in terms of the target observable. The Bessel–Gaussian limit invoked for v3{4}/v3{2} (Fig. 4) is an independent statistical expectation for fluctuation-dominated triangular flow, not an input derived from the measurement. Self-citations in the paper are standard references to ALICE detector upgrades, previous ALICE flow measurements, and technical frameworks; none of these is used to justify the central discrepancy claim in a load-bearing way. The reviewer concern about the absence of a closure test or centrality-estimator cross-check for the most central bins is a legitimate experimental-systems and systematic-uncertainty issue, but it is a correctness risk, not a circularity. No equation is reused by construction, no fitted parameter is renamed as a prediction, and no uniqueness theorem is imported from the authors' prior work. Therefore the circularity score is 0.

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

No free parameters are fitted in this contribution, and no new entities are introduced. The claims rest on standard cumulant analysis, the Trajectum model as an external baseline, and the Bessel-Gaussian approximation, all treated as assumptions here.

assumptions (4)

domain assumption Two-particle cumulants with |Delta eta|>1 separate collective flow from nonflow contributions.
Standard heavy-ion analysis assumption; the extraction of v2{2} and v3{2} in Section 2 depends on this separation.
domain assumption Trajectum is an adequate baseline; its Bayesian-tuned parameters are not validated within this paper.
The discrepancy claim is defined as data minus Trajectum; if the model is misspecified, the discrepancy would not imply incomplete initial-state modeling.
domain assumption The Bessel-Gaussian limit, sigma_v3/<v3> ~ sqrt(4/pi-1), is the correct baseline for v3{4}/v3{2}.
Used in Section 3, Fig. 4 to conclude triangular flow is fluctuation-dominated; it assumes v3 follows a Bessel-Gaussian distribution with no coherent component.
domain assumption Detector efficiency and acceptance corrections from the LHC24g3 Monte Carlo remove occupancy-dependent biases.
Section 2; the ultra-central result relies on these corrections, and no validation is shown in the text.

how reviews work

0 comments

Cite this review

Pith. "Pith review of Anisotropic Flow in Ultra-Central Pb$-$Pb Collisions at $\sqrt{\mathrm{s_{NN}}}=5.36$ TeV with ALICE." pith.science (2026). https://pith.science/paper/MM4GM75G

@misc{pith2026260811199,
  author       = {Pith},
  title        = {Pith review of: Anisotropic Flow in Ultra-Central Pb$-$Pb Collisions at $\sqrt\mathrms_NN=5.36$ TeV with ALICE},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MM4GM75G}},
  note         = {Machine review of arXiv:2608.11199}
}

abstract

Anisotropic flow measurements in heavy-ion collisions are sensitive to the spatial distribution of the initial state, and quark-gluon plasma transport properties such as the shear viscosity to entropy density ratio $(\eta/s)$. State-of-the-art relativistic hydrodynamic models successfully describe such flow measurements over a wide centrality range. However, the hydrodynamic description of anisotropic flow deviates from the experimental data in ultra-central collisions (UCC), where the average geometric anisotropy of the system becomes small and fluctuations dominate the initial-state geometry. This discrepancy constitutes the UCC puzzle, as the expected hierarchy of flow harmonics is not fully reproduced by current modeling approaches. Probing towards ultra-central collisions, effects on flow fluctuations due to the initial spatial anisotropies are suppressed. The measured flow can be explained by quantum fluctuations on the energy distribution of $^{208}$Pb nuclei. An octupole deformation of the $^{208}$Pb nuclei has been proposed as a remedy to improve the modeling of the measured $v_{3} \{2\} / v_{2} \{2\}$ ratio. In this contribution, we present measurements of $v_{3}\{2\}/v_{2}\{2\}$ ratio in ultra-central Pb\textendash Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36$ TeV with ALICE Run 3 detector and compare them with recent hydrodynamic model calculations.

Figures

Figures reproduced from arXiv: 2608.11199 by the authors.

**Figure 4.** Ratio v3{4}/v3{2} measured with |∆η| > 1 in Pb–Pb collisions at 0–5% centrality. The dashed line indicates the Bessel–Gaussian limit. 4. Summary In summary, the measurements presented in this work highlight the evolving role of the initial-state geometry from central to ultra-central Pb–Pb collisions. In the 0–5% centrality range, the collision geometry is still predominantly elliptic, reflecting the average overla… view at source ↗

Review history (2 revisions) →

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

14 extracted references · 4 canonical work pages

[1]

Ollitrault, Anisotropy as a signature of transverse collective flow, Phys

J.-Y . Ollitrault, Anisotropy as a signature of transverse collective flow, Phys. Rev. D 46 (1992) 229–245.doi: 10.1103/PhysRevD.46.229

doi:10.1103/physrevd.46.229 1992
[2]

Heinz, R

U. Heinz, R. Snellings, Collective flow and viscosity in relativistic heavy-ion collisions, Annual Review of Nuclear and Particle Science 63 (2013) 123–151.arXiv:1301.2826

arXiv 2013
[3]

V oloshin, Y

S. V oloshin, Y . Zhang, Flow study in relativistic nuclear collisions by fourier expansion of azimuthal particle distributions, Z. Phys. C 70 (1996) 665–672.arXiv:hep-ph/9407282

arXiv 1996
[4]

ALICE Collaboration, Anisotropic flow of charged particles in Pb–Pb collisions at √sNN =5.02 TeV, Phys. Rev. Lett. 116 (13) (2016) 132302.arXiv:1602.01119

arXiv 2016
[5]

A. V . Giannini, M. N. Ferreira, M. Hippert, D. D. Chinellato, G. S. Denicol, M. Luzum, J. Noronha, T. Nunes da Silva, J. Takahashi, Assessing the ultracentral flow puzzle in hydrodynamic modeling of heavy-ion collisions, Phys. Rev. C 107 (4) (2023) 044907.arXiv:2209.14001

work page Pith review arXiv 2023
[6]

Characterizing the many-body localization transition through correlations

P. Carzon, S. Rao, M. Luzum, M. Sievert, J. Noronha-Hostler, Possible octupole deformation of 208Pb and the ultracentralv 2 tov 3 puzzle, Phys. Rev. C 102 (5) (2020) 054905.arXiv:2007.06586

work page Pith review arXiv 2020
[7]

arXiv:2401.13727

ALICE Collaboration, ALICE upgrades during the LHC long shutdown 2, JINST 19 (05) (2024) P05062. arXiv:2401.13727

arXiv 2024
[8]

ALICE Collaboration, Technical design report for the upgrade of the ALICE inner tracking system, J. Phys. G 41 (8) (2014) 087002.arXiv:1402.4476

arXiv 2014

Show all 14 references

[9]

ALICE Collaboration, Technical design report for the upgrade of the ALICE time projection chamber, Technical Design Report CERN-LHCC-2013-020, ALICE-TDR-016, CERN, Geneva (Jan. 2014). URLhttps://cds.cern.ch/record/1622286

2013
[10]

ALICE Collaboration, Upgrade of the ALICE readout and trigger system, Technical Design Report CERN- LHCC-2013-019, ALICE-TDR-015, CERN, Geneva (Sep. 2013). URLhttps://cds.cern.ch/record/1603472

2013
[11]

W. H. Trzaska, et al., New fast interaction trigger for ALICE, Nucl. Instrum. Meth. A 845 (2017) 463–466. doi:10.1016/j.nima.2016.05.016

2017 doi
[12]

ALICE Collaboration, Technical design report for the upgrade of the online–offline computing system, Technical Design Report CERN-LHCC-2015-006, ALICE-TDR-019, CERN, Geneva (Mar. 2015). URLhttps://cds.cern.ch/record/2011297

2015
[13]

Bilandzic, C

A. Bilandzic, C. H. Christensen, K. Gulbrandsen, A. Hansen, Y . Zhou, Generic framework for anisotropic flow analyses with multiparticle azimuthal correlations, Phys. Rev. C 89 (6) (2014) 064904.arXiv:1312.3572

2014 arXiv
[14]

Y . Zhou, X. Zhu, P. Li, H. Song, Investigation of possible new features of anisotropic flow in relativistic heavy- ion collisions, Phys. Rev. C 91 (6) (2015) 064908.arXiv:1501.06992. 5

2015 arXiv

Pith tools

Reviewed August 15, 2026 · model on record in the stance chip above.

[1] [1]

Ollitrault, Anisotropy as a signature of transverse collective flow, Phys

J.-Y . Ollitrault, Anisotropy as a signature of transverse collective flow, Phys. Rev. D 46 (1992) 229–245.doi: 10.1103/PhysRevD.46.229

doi:10.1103/physrevd.46.229 1992

[2] [2]

Heinz, R

U. Heinz, R. Snellings, Collective flow and viscosity in relativistic heavy-ion collisions, Annual Review of Nuclear and Particle Science 63 (2013) 123–151.arXiv:1301.2826

arXiv 2013

[3] [3]

V oloshin, Y

S. V oloshin, Y . Zhang, Flow study in relativistic nuclear collisions by fourier expansion of azimuthal particle distributions, Z. Phys. C 70 (1996) 665–672.arXiv:hep-ph/9407282

arXiv 1996

[4] [4]

ALICE Collaboration, Anisotropic flow of charged particles in Pb–Pb collisions at √sNN =5.02 TeV, Phys. Rev. Lett. 116 (13) (2016) 132302.arXiv:1602.01119

arXiv 2016

[5] [5]

A. V . Giannini, M. N. Ferreira, M. Hippert, D. D. Chinellato, G. S. Denicol, M. Luzum, J. Noronha, T. Nunes da Silva, J. Takahashi, Assessing the ultracentral flow puzzle in hydrodynamic modeling of heavy-ion collisions, Phys. Rev. C 107 (4) (2023) 044907.arXiv:2209.14001

work page Pith review arXiv 2023

[6] [6]

Characterizing the many-body localization transition through correlations

P. Carzon, S. Rao, M. Luzum, M. Sievert, J. Noronha-Hostler, Possible octupole deformation of 208Pb and the ultracentralv 2 tov 3 puzzle, Phys. Rev. C 102 (5) (2020) 054905.arXiv:2007.06586

work page Pith review arXiv 2020

[7] [7]

arXiv:2401.13727

ALICE Collaboration, ALICE upgrades during the LHC long shutdown 2, JINST 19 (05) (2024) P05062. arXiv:2401.13727

arXiv 2024

[8] [8]

ALICE Collaboration, Technical design report for the upgrade of the ALICE inner tracking system, J. Phys. G 41 (8) (2014) 087002.arXiv:1402.4476

arXiv 2014