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REVIEW 3 major objections 5 minor 56 references

Observation of partonic flow in proton-proton and proton-nucleus collisions

T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read Quark-level flow signature appears in proton-proton collisions

desk verdict First baryon-meson v2 grouping/splitting in small systems, but the partonic-flow conclusion leans on a template subtraction that the paper doesn't fully validate. read the letter →

arxiv 2411.09323 v1 pith:TXRYW7LE submitted 2024-11-14 nucl-ex hep-exnucl-th

ALICE Collaboration This is my paper · ORCID
classification nucl-exhep-exnucl-th PACS 25.75.Ld25.75.-q
keywords ellipticflowquark-gluonplasmaquarkcoalescencesmallcollisionsystemsproton-protoncollisionsproton-leadpartonictwo-particlecorrelations
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 the first observation of a baryon-meson splitting in elliptic flow at intermediate transverse momentum in high-multiplicity proton-proton and proton-lead collisions. The pattern, in which the flow coefficients of mesons cluster together and those of baryons cluster at higher values, matches what is seen in heavy-ion collisions and is expected when flow develops among quarks before they combine into hadrons. If correct, the result implies that a stage of collectively flowing partons exists in small collision systems, not just in large nuclei.

What carries the argument

The load-bearing mechanism is quark coalescence: when two or three flowing quarks combine to form a meson or baryon, the hadron inherits a $v_2$ determined by the summed $v_2$ of its constituent quarks, so baryons end up with a larger $v_2$ than mesons at the same $p_T$. The measurement technique is the long-range two-particle azimuthal correlation, with a large pseudorapidity gap between the correlated particles and a template fit that subtracts non-flow contributions using low-multiplicity events as a baseline.

What would settle it

A measurement of $v_2$ in the same high-multiplicity pp events using four-particle cumulants, which suppress non-flow, that does not show the baryon-meson grouping at $3 < p_T < 8$ GeV/c would undermine the claim; alternatively, a re-analysis with a multiplicity-dependent non-flow template from a controlled Monte Carlo without flow that removes the ~5$\sigma$ splitting would falsify the extraction.

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Extended reading notes

Core claim

The paper's central claim is that the measured elliptic flow $v_2$ of identified hadrons in high-multiplicity p-Pb and pp collisions shows the same grouping by quark number as in Pb-Pb collisions: at transverse momenta of roughly 3 to 8 GeV/c, mesons ($\pi^\pm$, $K^\pm$, $K^0_S$) share a common $v_2$ within about one standard deviation, baryons ($p+\bar{p}$, $\Lambda+\bar{\Lambda}$) share a higher $v_2$ within about one standard deviation, and the baryon-meson separation is about five standard deviations. This is the first time this pattern has been observed in small collision systems. The authors compare the data with a hybrid model that combines hydrodynamic flow of a quark-gluon plasma with quark-coalescence hadronization; this model reproduces the grouping and splitting, while a version without coalescence does not. They conclude that the systems created in high-multiplicity p-Pb and pp collisions include a stage with collectively flowing partons.

Load-bearing premise

The analysis assumes that high-multiplicity two-particle correlations are exactly a scaled copy of low-multiplicity correlations (the non-flow template) plus a pure flow term, so if non-flow contributions change with multiplicity, the extracted flow coefficients and the baryon-meson split could be biased.

Editorial extensions

If this is right

  • High-multiplicity proton-proton and proton-lead collisions at the LHC produce a system that contains a stage of collectively flowing partons, similar to heavy-ion collisions.
  • The observed grouping of meson $v_2$ values and baryon $v_2$ values at $p_T$ of 3–8 GeV/c is a new signature of quark-coalescence hadronization in small systems.
  • Models that include only hadronic rescattering or initial-state correlations fail to reproduce the baryon-meson splitting, so the data favor a partonic phase.
  • A hydrodynamic evolution followed by quark coalescence and fragmentation currently provides the best description of identified-hadron $v_2$ in both small and large collision systems.

Reading between the lines

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

  • A natural extension would be to look for the same baryon-meson $v_2$ grouping among charm hadrons, such as $D$ mesons, in high-multiplicity proton-proton collisions, since number-of-constituent-quark scaling predicts a specific offset from light hadrons.
  • If the interpretation is correct, four-particle cumulant measurements, which are less sensitive to non-flow, should reproduce the splitting at intermediate $p_T$; a null result there would directly challenge the partonic-flow conclusion.
  • The result, if confirmed, implies that the event generators used as heavy-ion baselines should incorporate a partonic phase in small systems rather than relying on pure string fragmentation.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The ALICE Collaboration presents pT-differential elliptic flow coefficients v2 for identified mesons (pi^pm, K^pm, K0S) and baryons (p+pbar, Lambda+Lambdabar) in high-multiplicity pp collisions at sqrt(s)=13 TeV and p-Pb collisions at sqrt(s_NN)=5.02 TeV, using two-particle correlations with a large pseudorapidity separation and a template-fit subtraction of non-flow contributions. The paper reports, for the first time, a baryon-meson grouping (within about 1 sigma) and splitting (about 5 sigma) at intermediate pT (about 3--8 GeV/c) in both small systems, similar to the pattern observed in Pb-Pb collisions. The data are compared with the Hydro-Coal-Frag model, which includes hydrodynamics, quark coalescence, and fragmentation, and with a Hydro-Frag model without coalescence; only the former reproduces the grouping and splitting. On this basis the authors conclude that the created system in high-multiplicity pp and p-Pb collisions includes a stage with collectively flowing partons.

Significance. If the observation is robust, this is the first identified-particle measurement of the baryon-meson v2 grouping and splitting in small collision systems, and it provides qualitatively new evidence in the debate about QGP-like collectivity in pp and p-Pb collisions. The paper has substantial experimental strengths: very large event samples, a Bayesian PID approach with high purity, a three-subevent correlation method with large |Delta-eta| gaps, and a systematic-uncertainty treatment that includes event-selection, track, PID, and template variations. The model comparison is also valuable, and the authors correctly note that the Hydro-Coal-Frag parameters were tuned to pT spectra rather than to v2, making the comparison semi-independent. The central limitation is that the v2 extraction, and hence the claimed grouping and splitting, rests on the template-fit ansatz of Eq. (A.1), whose validity against multiplicity-dependent non-flow is not fully demonstrated; this is the main load-bearing point that needs strengthening before the claim can be accepted at face value.

major comments (3)
  1. [Section A.4, Eq. (A.1)] The template-fit method assumes that the high-multiplicity (HM) correlation function is a scaled low-multiplicity (LM) non-flow template plus a flow modulation, i.e. that all non-flow correlations have the same shape in HM and LM and differ only by a global scale F. The LM and HM event classes used here are not matched in Nch (LM requires Nch<20, HM requires Nch>25 and yields <Nch>~35), and if jet-fragmentation or resonance-decay correlations per particle grow with multiplicity in a species-dependent way, a single scale F cannot absorb the shape difference and the extracted v2 will be biased. The paper's estimate that non-flow contributes less than 1% for pT>0.6 GeV/c comes from applying the template fit to PYTHIA 8, a generator without collective flow; this cannot validate the separation in real data where a flow component is also present. The paper's own observation that string-shoving PYTHIA produces negative v2 after the template fit shows that the extracted v2 is sensitive to the detailed shape and normalization of the template, which makes the concern concrete. I would like the authors to provide a closure test in which a known multiplicity- and species-dependent non-flow contribution is embedded in a flow-dominated sample and shown not to generate a spurious baryon-meson splitting, or to demonstrate the stability of the grouping and splitting with an alternative estimator such as four-particle cumulants.
  2. [Main text, Figs. 3--4 and conclusion] The concluding claim that 'the system created in high-multiplicity p-Pb and pp collisions includes a stage with collectively flowing partons' is presented as the main physics conclusion, but the supporting model comparison with Hydro-Coal-Frag is only qualitative. The paper does not report a goodness-of-fit measure, nor does it quantify the level of agreement between the model and the measured v2 points, apart from visual inspection. More importantly, the claim of a '~5 sigma' splitting between baryons and mesons is not defined: it is not stated whether the significance is statistical only, whether systematic uncertainties are included, how the average over pT bins is performed, or which pairs of species are compared. Because the splitting is the key observable, the authors should give a precise definition of the significance and, if systematics are excluded, show explicitly how the significance changes when the systematic uncertainties from the template variation are included.
  3. [Section A.1 and A.4] The LM template is built from events with Nch<20, while the HM signal is measured at <Nch>~35, and the V0A/V0C multiplicity classes and FMD acceptances differ between pp and p-Pb. The paper should show, or at least summarize, the actual one-dimensional Delta-phi projections of the HM and LM correlation functions before and after the template scaling F, for at least one representative species and pT interval. Without such a visual or quantitative check, the reader cannot assess whether the assumed shape universality of the non-flow component is plausible, and whether the template fit is dominated by the flow term or by the F-scaled non-flow term at intermediate pT.
minor comments (5)
  1. [Figure 2] The caption of Fig. 2 repeats the phrase 'mesons (pi^pm, K^pm, K0S) and baryons (p+pbar, Lambda+Lambdabar)' in a way that is difficult to parse; please reformulate the caption to state once which symbols denote which species.
  2. [Eq. (A.2)] The combination v2^{PID} = sqrt(V2Delta^{TPC-FMD1,2} V2Delta^{TPC-FMD3} / V2Delta^{FMD1,2-FMD3}) should be accompanied by a brief statement of the underlying assumptions, in particular that the three V2Delta values share a common reference flow, otherwise the square-root formula is not transparent to the reader.
  3. [Abstract and introduction] The phrase 'within ~1 sigma' for the grouping should be made quantitative: specify over which pT range and for which species pairs the compatibility is evaluated, noting that the pp measurement extends only to about 6 GeV/c while the p-Pb and Pb-Pb measurements reach 10 GeV/c.
  4. [Section A.1] In the event-selection description, the pp HM class is given as '0.07% V0M' with Nch>25; the same quantity appears in the Fig. 2 caption as '0.07% V0M (-0' which appears truncated. Please correct the caption and ensure all multiplicity/centrality classes are printed consistently.
  5. [Main text, Figs. 3--4] The comparison with the Hydro-Coal-Frag and Hydro-Frag models is shown with statistical uncertainties only; the text should state explicitly whether model uncertainties or parameter variations are included, since the claim that Hydro-Coal-Frag provides 'the best possible description' depends on the robustness of the model curves.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the v2 measurement is an empirical extraction, the model comparison is a genuine prediction, and no load-bearing step reduces to its own input by equation or by self-citation.

full rationale

The paper's central result is a measured anisotropy pattern, not a quantity derived from a fitted parameter. The v2 values come from two-particle correlation functions using Eq. A.1 with an empirically measured low-multiplicity template; the statement that HM correlations equal a scaled LM template plus a flow modulation is an explicit assumption of the extraction method, and the extracted VnDelta is a data-driven Fourier coefficient, not an identity that forces the baryon-meson grouping. The non-flow residual estimate is a PYTHIA closure test, which is a check on the method rather than an input to it. The Hydro-Coal-Frag comparison is a genuine prediction: the model parameters are tuned to identified-hadron pT spectra (Ref. [47]) and, for pp, to a different multiplicity interval, and the v2 curves are then compared with the data; no v2 information is used in the tuning. Refs. [47,48] are external theory papers by the Song/Zhao group, and the ALICE collaboration's use of them is a normal external-model comparison, not a self-citation chain. The interpretation of baryon-meson grouping as evidence for partonic collectivity inherits the standard heavy-ion phenomenology; it is an inductive inference with alternatives considered (UrQMD, PYTHIA string-shoving, AMPT), not a definitional equivalence. No equation in the paper reduces a claimed prediction to its own input, so no circular step can be exhibited.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The observation depends on the template-fit ansatz with free parameters F and G, and on standard assumptions about non-flow suppression. The partonic-flow interpretation additionally relies on the quark-coalescence framework and the validity of the Hydro-Coal-Frag model.

free parameters (3)
  • F (template fit scaling factor) = fitted per event class and correlation function
    In Eq. A.1, F scales the low-multiplicity non-flow template; its fitted value directly affects the extracted VnDelta and thus the final v2.
  • G (template fit flow modulation factor) = fitted per event class and correlation function
    In Eq. A.1, G scales the flow modulation term; the extracted v2 depends on this fit.
  • Hydro-Coal-Frag model parameters = tuned to pT spectra in refs [47,48]
    The model comparison relies on externally tuned parameters (e.g., initial conditions, viscosity, coalescence probability) from previous work; they are not fitted to v2 in this paper, but the interpretation of partonic flow depends on the model's validity.
assumptions (4)
  • domain assumption High-multiplicity events are a linear superposition of low-multiplicity non-flow events plus a flow modulation.
    Eq. A.1 underlies the template-fit extraction; if HM non-flow differs from LM, the v2 results could be biased.
  • domain assumption Large pseudorapidity separation (1.1 < |Delta eta| < 7.8 in p-Pb, 1.1 < |Delta eta| < 6.4 in pp) suppresses non-flow from jets and decays.
    Used in Section A.4 to justify the two-particle correlation method as a flow measurement.
  • domain assumption Baryon-meson v2 grouping at intermediate pT is a signature of quark coalescence in a partonic medium.
    Interpretive framework established in heavy-ion literature (refs [14,17-19]) and applied here to small systems.
  • domain assumption The Hydro-Coal-Frag model provides a reliable description of hadron spectra and flow when tuned to pT spectra.
    The conclusion of partonic flow relies on this model's successful description of the data and the failure of alternatives.

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Cite this review

Pith. "Pith review of Observation of partonic flow in proton-proton and proton-nucleus collisions." pith.science (2026). https://pith.science/paper/TXRYW7LE

@misc{pith2026241109323,
  author       = {Pith},
  title        = {Pith review of: Observation of partonic flow in proton-proton and proton-nucleus collisions},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TXRYW7LE}},
  note         = {Machine review of arXiv:2411.09323}
}
read the original abstract

Quantum Chromodynamics predicts a phase transition from ordinary hadronic matter to the quark-gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are not confined within hadrons. The QGP is generated in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the anisotropic azimuthal expansion of the produced matter, provide a unique tool to unravel QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum, a feature associated with flow being imparted at the quark level, confirming the existence of the QGP. The observation of QGP-like features in relativistic proton-proton and proton-ion collisions has sparked debate about possible QGP formation in smaller collision systems, which remains unresolved. In this article, we demonstrate for the first time the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton-lead and proton-proton collisions at the Large Hadron Collider (LHC). These results are described by a model that includes hydrodynamic flow followed by hadron formation via quark coalescence, replicating features observed in heavy-ion collisions. This observation is consistent with the formation of a partonic flowing system in proton-proton and proton-lead collisions at the LHC.

Figures

Figures reproduced from arXiv: 2411.09323 by the authors.

Figure 1
Figure 1. A schematic representation of the overlap region in a collision is shown in gray, along with overall par￾ticle emission patterns in the transverse (x-y) plane, represented by large arrows. a) Non-flow sources: These are independent emissions, such as those from resonance decays or jets, where jets are collimated streams of hadrons created when a high-energy quark or gluon fragments after a collision. These effects l… view at source ↗
Figure 2
Figure 2. Left: pT-differential v2 measured with the two-particle correlation method [40] for mesons (π ±, K±, K0 S ) and baryons (p+p, Λ+Λ) in semicentral Pb–Pb collisions at √ sNN = 5.02 TeV. Middle: pT-differential v2 measured with the two-particle correlation method for mesons (π ±, K±, K0 S ) and baryons (p+p, Λ+Λ) in high-multiplicity p–Pb collisions at √ sNN = 5.02 TeV. Right: Same for pp collisions at √ s = 13 TeV. ⟨N… view at source ↗
Figure 3
Figure 3. pT-differential v2 measured with two-particle correlation for mesons (π ±, K±, K0 S ) and baryons (p+p, Λ+Λ) in high-multiplicity p–Pb collisions at √ sNN = 5.02 TeV. Comparisons with the calculations from the Hydro￾Coal-Frag model (left) and the Hydro-Frag model (right) are also presented [47, 48]. Only statistical uncertainties are shown for the calculations. 0 5 10 (GeV/c) T p 0 0.05 0.1 0.15 0.2 0.25 | < 6.4} η∆… view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: pT-differential v2 measured with two-particle correlation for mesons (π ±, K±, K0 S ) and baryons (p+p, Λ+Λ) in high-multiplicity pp collisions at √ s = 13 TeV. Comparisons with the calculations from the Hydro-Coal￾Frag model (left) and the Hydro-Frag model (right) are…

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Reviewed August 12, 2026 · model on record in the stance chip above.