Investigating charm-quark dynamics in the QGP via the charm-hadron elliptic flow in $\rm{Pb-Pb}$ collisions with ALICE

Chuntai Wu (for the ALICE Collaboration)

REVIEW 3 major objections 5 minor 12 references

Investigating charm-quark dynamics in the QGP via the charm-hadron elliptic flow in $\rm{Pb-Pb}$ collisions with ALICE

T0 review · 3 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read The paper reports the first LHC measurement of prompt and non-prompt $\Lambda_c^+$ baryon elliptic flow and a 3.6$\sigma$ baryon-meson splitting at $p_T>4$ GeV/$c$.

desk verdict First LHC Lambda_c v2 is a real step forward, but the '3.6 sigma first evidence' splitting claim outruns what the proceedings actually define. read the letter →

arxiv 2509.02502 v1 pith:VYVHCSMR submitted 2025-09-02 nucl-ex hep-ex

Chuntai Wu (for the ALICE Collaboration) This is my paper · ORCID

classification nucl-exhep-ex PACS 25.75.-q25.75.Ld

keywords ellipticflowcharmquarkLambda_cbaryonquark-gluonplasmabaryon-mesonsplittingPb-Pbcollisionshadronizationheavy-ion

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

These proceedings report the ALICE measurement of the elliptic flow ($v_2$) of charm hadrons in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV from Run 3, including the first-ever LHC measurements of prompt and non-prompt $\Lambda_c^+$ baryon $v_2$. The central result is that for $p_T>4$ GeV/$c$, prompt $\Lambda_c^+$ baryons have a larger $v_2$ than prompt $D^0$ mesons by 3.6 standard deviations, described as the first evidence of baryon-meson splitting in the charm sector. If the effect is real, charm quarks do not hadronize in isolation; they acquire extra collective motion when they form baryons, most naturally through quark coalescence in the quark-gluon plasma. The paper also reports the first prompt $D^0$ $v_2$ measurement below 1 GeV/$c$, a low-$p_T$ mass ordering relative to light flavor, and a 2.1$\sigma$ hint that $D_s^+$ $v_2$ is smaller than the non-strange D-meson $v_2$.

What carries the argument

The analysis chains the scalar-product $v_2$ estimator, a multi-class boosted-decision-tree classifier trained following the Ref. [5] strategy, simultaneous fits to the invariant-mass and $v_2$-versus-mass distributions, and a data-driven estimate of the feed-down fraction, followed by a linear fit in feed-down fraction that separates prompt and non-prompt $v_2$ components. The load-bearing part is the prompt/non-prompt separation: the claimed 3.6$\sigma$ splitting compares prompt $\Lambda_c^+$ with prompt $D^0$, so any leakage of beauty-decay candidates, which have smaller $v_2$, into the prompt sample would artificially enhance the splitting.

What would settle it

A Monte Carlo closure test would settle this: run the same analysis on simulated Pb--Pb events with a known feed-down fraction and check whether the extracted prompt $\Lambda_c^+$ $v_2$ at $p_T>4$ GeV/$c$ matches the true value within the quoted uncertainty. An independent decay-length-based tagger should also reproduce the same 3.6$\sigma$ splitting; if it does not, the BDT-based separation is the likely source.

Watch

Extended reading notes

Core claim

The paper's central claim is that in 30--50% central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV, prompt $\Lambda_c^+$ baryons have a larger elliptic flow than prompt $D^0$ mesons for $p_T>4$ GeV/$c$, with a significance of 3.6$\sigma$. This is presented as the first evidence of baryon-meson splitting in the charm sector. The non-prompt (beauty-decay) $\Lambda_c^+$ and $D^0$ have compatible $v_2$ values that are smaller than their prompt counterparts, which the authors attribute to the larger beauty-quark mass and its longer relaxation time in the medium. The measurements also show a mass ordering at low $p_T$ between charm and light-flavor hadrons, a flat or decreasing trend at intermediate $p_T$, and convergence of different species for $p_T>10$ GeV/$c$, which the authors connect to path-length-dependent parton energy loss.

Load-bearing premise

The 3.6-$\sigma$ baryon-meson splitting stands on the assumption that the classifier and feed-down correction cleanly separate prompt $\Lambda_c^+$ baryons from those produced in beauty decays; if even a modest fraction of the smaller-flow non-prompt baryons leaks into the prompt sample, the splitting could be an artifact.

Editorial extensions

If this is right

If the effect is real, charm baryons will be confirmed to acquire stronger collective flow than charm mesons at $p_T>4$ GeV/$c$ when the larger 2024--2025 data sample is analyzed.
The ordering baryon $>$ meson, with strange $D_s^+$ slightly below non-strange $D$ mesons, gives hadronization models a multi-species benchmark that transport-plus-coalescence calculations must reproduce simultaneously.
The smaller $v_2$ of non-prompt charm hadrons relative to prompt ones supports the picture that beauty quarks thermalize more slowly than charm quarks in the quark-gluon plasma.
The convergence of charm and light-flavor $v_2$ above 10 GeV/$c$ indicates that at high momentum the flow signal is dominated by path-length-dependent parton energy loss rather than by hadronization.
The centrality dependence of the charm $v_2$ pattern provides a new handle on how collision eccentricity, QGP size, and QGP lifetime combine to set the final flow.

Reading between the lines

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

If the 3.6$\sigma$ splitting survives with more Run 3 data, a natural extension is a finer centrality scan of the $\Lambda_c^+$/$D^0$ $v_2$ ratio; coalescence models would predict the largest splitting in the centrality range where the partonic phase is longest.
The quoted significance was identified after scanning $p_T$ intervals, so a look-elsewhere correction over the scanned bins and particle species would clarify whether 3.6$\sigma$ is a single-bin fluctuation.
A coupled analysis of $\Lambda_c^+/D^0$ ratios and $v_2$ could distinguish recombination from fragmentation more sharply than either observable alone, because the two mechanisms make different joint predictions for yield and flow.
The same BDT plus feed-down machinery could be turned on beauty baryons, for which a baryon-meson splitting would be expected at lower transverse momentum because beauty quarks are heavier and thermalize later.

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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

You should know two things about arXiv:2509.02502. First, the measurements are genuinely new: first Lambda_c v2 at the LHC, first D0 v2 below 1 GeV/c, and new Run 3 data at 5.36 TeV. That is real progress and the analysis follows established ALICE methods. Second, the headline claim—'first evidence of baryon-meson splitting in the charm sector' at 3.6 sigma—is not backed up by the proceedings as written. The significance is undefined: no statement about whether systematics are included, no note on correlations between D0 and Lambda_c, and no trials factor for choosing the pT > 4 GeV/c window after looking at the data. The text itself says the splitting appears only above 4 GeV/c, so the threshold is post-hoc. That makes the evidence claim weaker than the number alone suggests.

What the paper does well is straightforward: cleanly reports new v2 measurements for several charm hadrons in 30–50% centrality, compares them to light-flavor results and models, and notes where models deviate. The model comparisons are qualitative, but that is normal for a proceedings. The feed-down handling follows the previous ALICE non-prompt D0 analysis, with a data-driven fraction; the reference is appropriate. On the direction of possible bias, the stress-test note is right: non-prompt contamination would likely dilute the prompt Lambda_c excess, not create it, so the splitting may survive better than the 3.6 sigma itself implies. Still, a bias in the feed-down correction could move the difference either way.

The citation pattern looks clean—no self-citation inflation, the key methodological reference is the earlier ALICE paper. The abstract and text are honest about what is measured. The soft spots are the significance definition, the selected pT interval, and the absence of uncertainty breakdowns. For a proceedings, some of that is expected, but the phrase 'first evidence' elevates the claim beyond what the current write-up supports.

Who gets value: heavy-ion phenomenologists working on charm transport and hadronization. This is exactly the kind of measurement that constrains coalescence models. It deserves a serious referee. My recommendation: send it to peer review, but expect the collaboration to either define the significance with full systematics and a trials factor, or soften the 'evidence' language to 'hint' or 'indication.' The measurement itself is solid enough to publish; the interpretation needs calibration.

Referee Report

3 major / 5 minor

Summary. The manuscript is a proceedings contribution from the ALICE collaboration reporting elliptic flow (v2) measurements of charm hadrons in Pb-Pb collisions at sqrt(s_NN) = 5.36 TeV from Run 3 data. It presents v2 of prompt D0, D+, Ds+ mesons in several centrality classes and, for the first time at the LHC, prompt and non-prompt Lambda_c+ baryons in the 30-50% centrality class. The analysis uses the scalar-product method with BDT-based candidate selection and a data-driven feed-down correction. The central physics claim, stated in Section 4, is that at pT > 4 GeV/c the prompt Lambda_c+ v2 exceeds the prompt D0 v2 with a significance of 3.6 sigma, which is interpreted as the first evidence of baryon-meson splitting in the charm sector. The measurements are compared with several model predictions.

Significance. If the reported 3.6-sigma effect is robust, the paper delivers a genuinely new result: the first measurement of Lambda_c+ elliptic flow in heavy-ion collisions and a direct experimental handle on charm baryon-meson splitting, which is sensitive to coalescence hadronization and heavy-quark transport in the QGP. The paper's strengths include the use of a well-established scalar-product method, a mature data-driven feed-down procedure from a previous ALICE publication, and comparison with a broad set of model predictions. However, the headline evidence claim is currently undersupported because the paper does not define the quoted significance, does not specify whether systematic uncertainties are included in the displayed error bars, and does not address the post-hoc choice of the pT > 4 GeV/c window. These are load-bearing issues for the central conclusion, so the paper needs revision before the claim can be accepted as stated.

major comments (3)

[Section 4, Fig. 1 (right)] The central claim 'At pT > 4 GeV/c, the prompt Lambda_c+-baryon v2 is larger than that of D0 mesons with a significance of 3.6 sigma' is not supported by a defined significance calculation. Please specify whether the 3.6 sigma is based only on statistical uncertainties or on the full statistical-plus-systematic covariance, and explain how correlations between the D0 and Lambda_c+ measurements are handled. The manuscript currently gives no information on this point, which is essential for judging the claim.
[Section 4, pT range selection] The pT > 4 GeV/c interval appears to be chosen after inspecting the data, as the text first states that below 4 GeV/c the prompt Lambda_c+ and D0 v2 are compatible and then quotes the 3.6-sigma difference above that threshold. If multiple pT thresholds or intervals were scanned, the quoted local significance overstates the evidence. Please state whether the threshold was pre-specified, and if not, apply a look-elsewhere correction or report the trials factor.
[Section 2, feed-down procedure] The prompt and non-prompt separation relies on a multi-class BDT and a data-driven feed-down fraction following Ref. [5], but the manuscript does not quantify the systematic uncertainty associated with this correction. Since the non-prompt v2 is smaller than the prompt v2, simple leakage would dilute the difference, but a bias in the feed-down fraction itself could still affect the extracted prompt Lambda_c+ v2. Please state how the feed-down systematic is evaluated and whether it is included in the uncertainties shown in Fig. 1 (right).

minor comments (5)

[Section 1, reference [1]] Reference [1] (Liu and Liu, Quark-gluon plasma formation time and direct photons) does not appear to support the statement that elliptic flow originates mainly from the initial-state spatial asymmetry; please cite a more appropriate reference for this foundational statement.
[Section 2, wording] The phrase 'combining couples or triplets of tracks' should be 'combining pairs or triplets of tracks'.
[Figures 1 and 2] The figure captions do not specify whether the error bars represent statistical uncertainties only or total uncertainties. Please add this information to each caption.
[Section 3] The text highlights the first measurement of prompt D0 v2 at pT < 1 GeV/c but does not comment on possible low-pT acceptance or efficiency corrections; a brief statement on this would be helpful.
[Section 5] The summary mentions future constraints from RAA and Run 3 data, but it would be useful to also mention whether the current v2 measurements are already compatible with the model predictions in a quantitative sense, rather than only qualitatively.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the v2 measurements are extracted from data with standard methods and compared to independent model predictions.

full rationale

The paper reports an experimental measurement rather than a derivation. The v2 values are extracted from data using the scalar-product method and simultaneous fits to the invariant mass spectrum and v2 as a function of mass. The prompt and non-prompt separation follows an existing ALICE analysis (Ref. [5]) for BDT training and feed-down estimation. That prior publication is an independent measurement of non-prompt D0-meson v2 and is not constructed from the present paper's results, so relying on it is methodological reuse rather than a circular input. The model comparisons use external published calculations (Refs. [7-12]) whose assumptions do not include the measured v2 values. The central claim of a 3.6-sigma baryon-meson splitting is a data-level comparison, not a fitted parameter renamed as a prediction. No equation in the paper reduces a predicted quantity to an input by construction. The undefined significance and the lack of a trials-factor correction are statistical-correctness concerns, not circularity, and do not affect the circularity score beyond a minimal note.

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

The paper is a measurement and introduces no free parameters or invented entities. It relies on standard flow-analysis assumptions (scalar-product method, BDT prompt/non-prompt separation, data-driven feed-down estimate) that are referenced to prior ALICE work but not fully documented in the proceedings.

assumptions (3)

domain assumption The scalar-product method with |Delta eta| > 1.3 correctly measures the correlation with the symmetry plane.
Used in Section 2 to extract v2; standard in heavy-ion flow analyses but still an assumption that non-flow contributions are suppressed.
domain assumption The multi-class BDT provides an unbiased separation of prompt and non-prompt contributions.
Section 2: the classifier is trained following Ref. [5]; its performance is not quantified in these proceedings, so the separation is an assumption.
domain assumption The data-driven feed-down fraction estimate is accurate.
Section 2: the feed-down fraction is estimated with a data-driven method; a bias would directly affect the prompt v2 decomposition.

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Cite this review

Pith. "Pith review of Investigating charm-quark dynamics in the QGP via the charm-hadron elliptic flow in $\rm{Pb-Pb}$ collisions with ALICE." pith.science (2026). https://pith.science/paper/VYVHCSMR

@misc{pith2026250902502,
  author       = {Pith},
  title        = {Pith review of: Investigating charm-quark dynamics in the QGP via the charm-hadron elliptic flow in $\rmPb-Pb$ collisions with ALICE},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VYVHCSMR}},
  note         = {Machine review of arXiv:2509.02502}
}

abstract

In these proceedings, we report the measurement of the elliptic flow ($v_{\rm 2}$) of prompt and non-prompt $\mathrm{D^0}$, prompt $\mathrm{D^+}$ and $\mathrm{D_s^+}$ mesons, and, for the first time ever at the LHC, prompt and non-prompt $\Lambda_\mathrm{c}^+$ baryons in \PbPb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36\ \mathrm{TeV}$ collected by the ALICE experiment during the LHC Run 3. The analysis is performed at midrapidity ($|y| < 0.8$) in different centrality classes ($30-40\%$, $40-50\%$, and $60-80\%$ for $\mathrm{D^0}$, $\mathrm{D^+}$, and $\mathrm{D_s^+}$; $30-50\%$ for $\Lambda_\mathrm{c}^+$). The candidates are reconstructed from hadronic decay channels, and the $v_{\rm 2}$ is extracted via the scalar-product method. The measurements are compared to light-flavor results and several predictions of models that differently describe heavy-quark transport and hadronization processes in the QGP.

Figures

Figures reproduced from arXiv: 2509.02502 by the authors.

**Figure 1.** (left) shows the v2 of prompt D0 , D+ , and D+ s mesons in the 30-50% centrality interval, compared to that of inclusive J/ψ in the same centrality interval and of π + [6] in 30- 40%. This is the first measurement of the prompt D0 meson v2 at the transverse momentum pT < 1 GeV/c. Overall, the v2 of D+ mesons shows a good agreement with that of D0 mesons. At pT < 4 GeV/c, a mass ordering is observed when comparing v2… view at source ↗

**Figure 2.** Measured v2 of prompt D0 , D+ , and D+ s mesons, in 30-40% (left), 40-50% (middle), and 60-80% (right), compared to that of π + . 4 Prompt and non-prompt Λ + c -baryon v2 The Λ + c baryon, composed of one charm quark and two light quarks, is expected to be more sensitive to the coalescence mechanism in QGP. As shown in [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Comparison of measured v2 of prompt Λ + c baryons and D0 mesons with model predictions. 5 Summary In these proceedings, the first measurements of D-meson and Λ + c -baryon v2 performed with ALICE using the 2023 Pb–Pb data collected in Run 3 were reported. In combination with [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

12 extracted references · 1 canonical work pages

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Pith tools

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

[1] [5]

Acharya et al

S. Acharya et al. (ALICE), Measurement of non-prompt D 0-meson elliptic flow in Pb–Pb collisions at √sNN = 5.02 TeV, Eur. Phys. J. C 83, 1123 (2023), 2307.14084. 10.1140/epjc/s10052-023-12259-3

arXiv 2023

[2] [1]

Liu, S.X

F.M. Liu, S.X. Liu, Quark-gluon plasma formation time and direct photons from heavy ion collisions, Phys. Rev. C 89, 034906 (2014), 1212.6587. 10.1103 /Phys- RevC.89.034906

arXiv 2014

[3] [2]

Poskanzer, S.A

A.M. Poskanzer, S.A. V oloshin, Methods for analyzing anisotropic flow in relativistic nuclear collisions, Phys. Rev. C 58, 1671 (1998), nucl-ex/9805001. 10.1103/Phys- RevC.58.1671

arXiv 1998

[4] [3]

Acharya et al

S. Acharya et al. (ALICE), ALICE upgrades during the LHC Long Shutdown 2, JINST 19, P05062 (2024), 2302.01238. 10.1088/1748-0221/19/05/P05062

arXiv 2024

[5] [4]

T. Chen, C. Guestrin, XGBoost: A Scalable Tree Boosting System (2016), 1603.02754. 10.1145/2939672.2939785

arXiv 2016

[6] [6]

Acharya et al

S. Acharya et al. (ALICE), Anisotropic flow of identified particles in Pb-Pb collisions at√sNN = 5.02 TeV, JHEP 09, 006 (2018), 1805.04390. 10.1007/JHEP09(2018)006

arXiv 2018

[7] [7]

M. He, R. Rapp, Hadronization and Charm-Hadron Ratios in Heavy-Ion Collisions, Phys. Rev. Lett. 124, 042301 (2020), 1905.09216. 10.1103/PhysRevLett.124.042301

arXiv 2020

[8] [8]

S. Li, W. Xiong, R. Wan, Relativistic Langevin dynamics: charm versus beauty, Eur. Phys. J. C 80, 1113 (2020), 2012.02489. 10.1140/epjc/s10052-020-08708-y

arXiv 2020