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REVIEW 4 major objections 5 minor 17 references

Collision energy and system size dependence of $p_{\mathrm{T}}$-differential radial flow fluctuations $v_{0}(p_{\mathrm{T}})$ at RHIC

T0 review · 4 major / 5 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read The paper reports the first measurements of the transverse-momentum-differential radial-flow fluctuation v0(pT) in Au+Au and O+O collisions, arguing that the normalized quantity v0(pT)/v0 is a universal hydrodynamic response, largely indepe

desk verdict New RHIC data on v0(pT) worth having, but the universality and bulk-viscosity claims outrun the non-flow control and the missing systematics. read the letter →

arxiv 2608.00190 v1 pith:QEA4TOFJ submitted 2026-07-31 nucl-ex

Zaining Wang This is my paper
classification nucl-ex
keywords radialflowfluctuationsmeantransversemomentumv0(pT)bulkviscosityquark-gluonplasmasmallcollisionsystemsbeamenergyscanhydrodynamicresponse
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 measures a new differential observable, v0(pT), which correlates event-by-event fluctuations of the transverse-momentum spectrum with fluctuations of the mean transverse momentum. The principal claim is that the normalized ratio v0(pT)/v0 factorizes across centralities and across large and small collision systems, revealing a common hydrodynamic response κ0(pT)/κ0 that is independent of the fluctuation amplitude. The paper also shows that the integral fluctuation v0 follows the same charged-particle multiplicity dependence in Au+Au and O+O collisions, suggesting that the magnitude of radial-flow fluctuations is set by event-by-event variations of the initial system size. These results are used to argue that v0(pT) can serve as a probe of radial collectivity and of QGP transport properties, especially bulk viscosity. A sympathetic reader cares because radial flow has been much harder to constrain differentially than anisotropic flow, and a universal response curve would give a clean handle on the isotropic expansion of the quark–gluon plasma.

What carries the argument

The central object is the normalized differential radial-flow fluctuation v0(pT)/v0, defined through the event-wise correlation between fractional spectral fluctuations δn(pT) and mean-transverse-momentum fluctuations δ[pT]. In the hydrodynamic picture this ratio equals κ0(pT)/κ0, separating the response of the expanding medium to isotropic size fluctuations from the amplitude of those fluctuations. The observable's key features are a characteristic zero crossing near ⟨[pT]⟩, a mass ordering for identified hadrons, and the claim that factorization across centralities and systems signals a universal response.

What would settle it

A decisive test is to remeasure v0(pT)/v0 in O+O collisions with a larger pseudorapidity gap or a three-subevent method: if the universal factorization across centralities and between Au+Au and O+O is broken once non-flow is further suppressed, the central claim fails. Alternatively, if a hydrodynamic calculation without bulk viscosity already reproduces the mid-pT pion data within uncertainties, the claimed sensitivity to ζ/s would be falsified.

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Extended reading notes

Core claim

The paper presents first measurements of the pT-differential radial-flow fluctuation v0(pT) in Au+Au collisions from 7.7 to 200 GeV per nucleon pair and in O+O collisions at 200 GeV. It finds that the integral fluctuation v0 displays a common dependence on charged-particle multiplicity in both large and small systems, and that the normalized ratio v0(pT)/v0 collapses onto a nearly universal curve for pT below about 3 GeV/c. This factorization is interpreted as the experimental signature of a hydrodynamic response function κ0(pT)/κ0 that is largely independent of the fluctuation amplitude. Identified-hadron measurements show a mass ordering with the zero crossing near each species' mean trans

Load-bearing premise

The result rests on the assumption that the two-subevent method with a pseudorapidity gap of 0.1 is sufficient to remove short-range non-flow correlations, which is hardest to guarantee in the small O+O system; if residual non-flow survives there, the common Nch scaling and factorization could be experimental artifacts rather than genuine hydrodynamic response.

Editorial extensions

If this is right

  • If the factorization is correct, v0(pT)/v0 can be used as a model-independent probe of the radial hydrodynamic response without needing to know the event-by-event fluctuation amplitude.
  • The observed sensitivity of the pion v0(pT)/v0 to bulk viscosity suggests that this observable can provide new constraints on ζ/s, one of the least constrained transport coefficients of the quark–gluon plasma.
  • The common Nch dependence in Au+Au and O+O supports the view that radial-flow fluctuations are controlled by the event-wise system size, extending the idea of hydrodynamic collectivity to small collision systems.
  • Proton and antiproton v0(pT) differences at low beam energies point to an increasing role of baryon transport in the high-baryon-density region, offering a new flow-based handle on the QCD phase diagram.
  • The universal response curve can serve as a benchmark for hydrodynamic models, allowing initial-state and transport effects to be disentangled more cleanly than with mean-pT fluctuations alone.

Reading between the lines

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

  • A natural extension of this result is to test the same normalized response in yet smaller systems, such as p+A or p+p collisions at matching multiplicities; if the universal curve holds there, it would strengthen the claim that radial expansion is collective even in proton-sized systems.
  • The factorization implied by v0(pT)/v0 suggests a practical recipe for data-driven modeling: extract κ0(pT)/κ0 from one centrality and predict the pT-differential radial response in all other centralities and systems with essentially no free parameters.
  • The mass ordering and the proton–antiproton differences at low energy indicate that v0(pT) carries information not only about the bulk medium but also about baryon transport; combining v0 with anisotropic-flow factorization could help separate geometric effects from baryon-stopping effects.
  • If the bulk-viscosity sensitivity is confirmed by more precise calculations, v0(pT) could become a complementary observable to the mean-pT fluctuation, yielding a two-dimensional constraint on ζ/s as a function of temperature and baryon chemical potential.
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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

4 major / 5 minor

Summary. The paper reports STAR measurements of the integral radial-flow fluctuation v0 and its pT-differential version v0(pT) in Au+Au collisions at sqrt(sNN)=7.7-200 GeV and in O+O collisions at 200 GeV. The central claims are: (i) v0 follows a common N_ch dependence in Au+Au and O+O, suggesting initial-size-driven fluctuations; (ii) the normalized quantity v0(pT)/v0 factorizes across centralities and systems, interpreted as a universal hydrodynamic response kappa0(pT)/kappa0; and (iii) identified-hadron mass ordering and viscous model comparisons show sensitivity to bulk viscosity. The paper presents STAR Preliminary figures and is written as a short proceedings-style report.

Significance. If the results hold, they would extend the v0(pT) program from the LHC to RHIC energies and provide a small-system bridge in O+O collisions, with implications for the hydrodynamic interpretation of radial flow fluctuations and for constraining the bulk viscosity of the quark-gluon plasma. The factorization of v0(pT)/v0 across centrality and system size, if genuine, is a striking scaling law. The paper is also timely given recent LHC results. However, the significance is conditional on the control of non-flow correlations and on a quantitative presentation of uncertainties, neither of which is currently provided.

major comments (4)
  1. [Section 2, Fig. 1(c), Fig. 2(b)] The only suppression of short-range non-flow is a two-subevent selection with |Delta eta|>0.1. This gap is much smaller than the |Delta eta|>1 typically required to suppress short-range correlations in flow measurements. In the low-multiplicity O+O system, jet fragments, resonance decays, and Bose-Einstein correlations can survive and produce a sign pattern (negative at low pT, positive at high pT) similar to the hydrodynamic expectation, since a high-pT particle from a jet can be correlated with a shift in the event mean pT. Because the common-N_ch scaling in Fig. 1(a) and the factorization in Fig. 1(c) rest critically on the O+O points, the manuscript must show an eta-gap scan, a p+p baseline, or a quantitative non-flow model estimate. Without such a control, the claimed universality could be an acceptance/non-flow effect rather than a hydrodynamic response.
  2. [Section 3.1, Section 3.2, Fig. 1(a), Fig. 1(c), Fig. 3] The central claims of a 'common curve', 'collapse', and 'factorization' are supported only by visual comparison. No quantitative compatibility test (e.g., chi2/ndf, residuals, or centrality-dependent deviations) is reported, and no systematic-uncertainty budget is given anywhere in the paper. The figures show STAR Preliminary data without error bars in most panels. For a first measurement whose title emphasizes collision-energy and system-size dependence, the absence of quantitative comparisons and systematic uncertainties prevents the reader from assessing whether the claimed universality is statistically significant. The beam-energy dependence in Fig. 3 is likewise described qualitatively as 'weak' or 'strong' without a numerical measure.
  3. [Section 3.3, Fig. 2(c)] The bulk-viscosity sensitivity claim is based on a visual comparison with a single model calculation (TRENTo+MUSIC+SMASH) for charged pions in 0-5% Au+Au collisions. The text states that ideal and shear-only calculations underpredict the mid-pT magnitude while adding bulk viscosity 'brings the prediction closer to data'. No model uncertainties (e.g., variations in initial state, shear viscosity, or the specific value of zeta/s) and no data uncertainties are shown. The conclusion that these results 'demonstrate sensitivity to bulk viscosity' is therefore overstated. A quantitative comparison, such as a chi2/ndf scan over zeta/s values, is needed to support this load-bearing claim.
  4. [Eq. (2), Section 2] The factorization statement v0(pT)/v0 = kappa0(pT)/kappa0 is presented as if it directly isolates the hydrodynamic response. The precise definition of v0(pT) in Eq. (1) is ambiguous because the equation is typeset without clear division bars. If v0(pT) already contains a factor of v0 in the denominator, then the ratio v0(pT)/v0 is not the amplitude-independent response without additional assumptions. The manuscript should state the exact definition, show explicitly how the Truncated?
minor comments (5)
  1. [Eq. (1)] Equation (1) is malformed: the fraction structure is missing, making it impossible to determine whether v0(pT) includes a dividing factor of v0. Please rewrite with unambiguous fractions and define n(pT) before Eq. (1).
  2. [Fig. 1 caption] The caption contains garbled text: '8±(Au+Au 60-70%) = 27part' and '1±(O+O 10-20%) = 20part'. Presumably Npart values with uncertainties; please correct.
  3. [Section 2] The notation 'ηgap=0.1' should be defined as |Delta eta|>0.1 between subevents, and the choice should be justified in context of the STAR acceptance.
  4. [Fig. 2(c)] The x-axis is labeled pT/<pT> but the text refers to pT without rescaling; state the rescaling explicitly in the caption and text.
  5. [References] Reference [13] contains an unusual DOI string '10.1103/y962-1lyg'; please verify it is correct and not a placeholder.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: v0(pT) is a measured observable and Eq. (2) is an interpretive label, not a fitted prediction.

full rationale

The paper's central claims are empirical measurements of the new observable v0(pT), defined directly from data in Eq. (1), with no parameters fitted to the factorization or scaling results. Equation (2), v0(pT)/v0 = κ0(pT)/κ0, is a notational identification of the measured normalized correlation with a hydrodynamic response quantity; it does not derive a prediction from an input and does not feed a fitted value back into the analysis. The factorization of v0(pT)/v0 across centralities and systems is presented as a data observation (Fig. 1c), not as an output forced by construction. The interpretation that this reflects a universal κ0(pT)/κ0 depends on external hydrodynamic models (Refs. [10,12]), which are independent references rather than self-citations. Model comparisons in Figs. 2(b,c) use published calculations and do not involve fitting the data. The possible inefficacy of the ηgap = 0.1 two-subevent method in O+O is a validity/correctness concern about non-flow contamination, not a circularity: it is an assumption about the measurement environment, not a step in which input and output reduce to each other. No fitted parameter is renamed as a prediction, no load-bearing self-citation is used, and no uniqueness theorem is imported from the authors' prior work. The derivation chain is self-contained: raw correlations yield v0 and v0(pT), and the physical response interpretation is an external model interpretation rather than a circular derivation.

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

No free parameters are introduced by this paper; the model comparisons use transport coefficients from external calculations, not fitted here. The main hidden load-bearing inputs are the interpretive equalities (Eq. 2), the non-flow suppression assumption, and the initial-size interpretation of the N_ch scaling.

assumptions (3)
  • domain assumption The measured v0(pT)/v0 equals the hydrodynamic response κ0(pT)/κ0 and is 'largely independent of the fluctuation amplitude' (Eq. 2).
    This is the central interpretive step: the ratio is assumed to isolate the medium response from the initial-state fluctuation amplitude. The paper cites [10,12] for this factorization rather than deriving it from STAR data.
  • domain assumption The two-subevent method with |Δη|>0.1 removes short-range non-flow sufficiently in both Au+Au and O+O.
    If residual non-flow survives in the small-system O+O data, the observed common N_ch scaling and factorization could be a detector/acceptance effect rather than a hydrodynamic response.
  • domain assumption Event-by-event fluctuations of the initial transverse size control v0; the 1/√N_ch trend is interpreted as independent particle emission from fluctuating sub-sources.
    The interpretation of the common N_ch curve as size-driven is not directly verified; this is a model interpretation.

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Cite this review

Pith. "Pith review of Collision energy and system size dependence of $p_{\mathrm{T}}$-differential radial flow fluctuations $v_{0}(p_{\mathrm{T}})$ at RHIC." pith.science (2026). https://pith.science/paper/QEA4TOFJ

@misc{pith2026260800190,
  author       = {Pith},
  title        = {Pith review of: Collision energy and system size dependence of $p_\mathrmT$-differential radial flow fluctuations $v_0(p_\mathrmT)$ at RHIC},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QEA4TOFJ}},
  note         = {Machine review of arXiv:2608.00190}
}
abstract

We report the first RHIC measurements of $v_{0}(p_\mathrm{T})$ in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=7.7$--$200$ GeV and O+O collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV. The integral fluctuation $v_0$ follows a common $N_{\rm ch}$ dependence in large and small systems, suggesting that the fluctuation magnitude is predominantly controlled by event-by-event fluctuations of the initial transverse size. The normalized response $v_0(p_\mathrm{T})/v_0$ factorizes across centralities and systems, revealing a universal hydrodynamic response $\kappa_0(p_\mathrm{T})/\kappa_0$ largely independent of the fluctuation amplitude. Identified-hadron mass ordering and viscous model comparisons demonstrate sensitivity to bulk viscosity ($\zeta/s$). These results establish $v_0(p_\mathrm{T})$ as a probe of the radial hydrodynamic response, collectivity across large and small collision systems, and QGP transport properties.

Figures

Figures reproduced from arXiv: 2608.00190 by the authors.

Figure 1
Figure 1. Integral and differential radial-flow fluctuations in Au+Au and O+O collisions at √ sNN = 200 GeV. Panel (a) shows the integral fluctuation strength v0 as a function of the charged-particle multiplicity Nch for inclusive charged hadrons and identified π ± , K ± , and p( ¯p). Panels (b) and (c) show, respectively, v0(pT) and the normalized quantity v0(pT)/v0 for inclusive charged hadrons in different centrality inter… view at source ↗
Figure 2
Figure 2. Identified-hadron radial-flow fluctuations and comparisons with hydrodynamic calculations in 0–5% central collisions at [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Beam-energy dependence of v0(pT) in Au+Au collisions. Panels (a)–(d) show v0(pT) for p, π + , ¯p and π − in 0–40% central Au+Au collisions at √ sNN = 9.2, 14.5, 19.6, 54.4, and 200 GeV. 2022YFA1604900, the National Natural Science Foundation of China (NSFC) under Contract Nos. 12025501 and 12547102, and the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Award No. DE￾SC0024602. Referen… view at source ↗

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Works this paper leans on

17 extracted references · 1 linked inside Pith

  1. [1]

    Heinz, R

    U. Heinz, R. Snellings, Collective flow and viscosity in relativistic heavy-ion collisions, Annu. Rev. Nucl. Part. Sci. 63 (2013) 123–151

    2013

  2. [2]

    Adamczyk, et al., Bulk properties of the medium produced in relativistic heavy-ion collisions from the beam energy scan program, Phys

    L. Adamczyk, et al., Bulk properties of the medium produced in relativistic heavy-ion collisions from the beam energy scan program, Phys. Rev. C 96 (2017) 044904

    2017

  3. [3]

    Chen, et al., Selected Highlights from STAR Experiment, Chin

    J. Chen, et al., Selected Highlights from STAR Experiment, Chin. Phys. Lett. 43 (3) (2026) 030102.arXiv:2601.12977

    arXiv 2026

  4. [4]

    STAR Collaboration, Imaging shapes of atomic nuclei in high-energy nuclear collisions, Nature 635 (8037) (2024) 67–72.arXiv:2401. 06625

    2024

  5. [5]

    STAR Collaboration, Imaging nuclear shape through anisotropic and radial flow in high-energy heavy-ion collisions, Rept. Prog. Phys. 88 (10) (2025) 108601.arXiv:2506.17785

    arXiv 2025

  6. [6]

    STAR Collaboration, Non-Monotonicity of Transverse Momentum Correlations in Au+Au Collisions at RHIC (4 2026).arXiv:2604. 06434

    2026

  7. [7]

    Wan, C.-Z

    J. Wan, C.-Z. Wang, Q.-Y . Shou, Y .-G. Ma, Tracing pT-differential radial flow from blast-wave analytics to quark coalescence, Phys. Rev. C 113 (6) (2026) 064902.arXiv:2509.24889

    arXiv 2026

  8. [8]

    S. Saha, R. Singh, B. Mohanty, pT-differential radial flow in a blast-wave model, Phys. Rev. C 112 (2) (2025) 024902.arXiv:2505.19697

    arXiv 2025

Show all 17 references
  1. [9]

    Schenke, C

    B. Schenke, C. Shen, D. Teaney, Transverse momentum fluctuations and their correlation with mean transverse momentum in heavy-ion collisions, Phys. Rev. C 102 (2020) 034905

    2020

  2. [10]

    Parida, R

    T. Parida, R. Samanta, J.-Y . Ollitrault, Probing collectivity in heavy-ion collisions with fluctuations of thep T spectrum, Phys. Lett. B 857 (2024) 138985

    2024

  3. [11]

    ALICE Collaboration, Long-range transverse momentum correlations and radial flow in Pb−Pb collisions at the LHC, Phys. Rev. Lett. 136 (3) (2026) 032302.arXiv:2504.04796

    2026

  4. [12]

    ATLAS Collaboration, Evidence for the Collective Nature of Radial Flow in Pb+Pb Collisions with the ATLAS Detector, Phys. Rev. Lett. 136 (3) (2026) 032301.arXiv:2503.24125

    2026

  5. [13]

    S. A. Jahan, H. Roch, C. Shen, Bayesian model selection and uncertainty propagation for beam energy scan heavy-ion collisions, Phys. Rev. C 113 (2) (2026) 024919.arXiv:2507.11394,doi:10.1103/y962-1lyg

    2026

  6. [14]

    Du, Characterizing radial flow fluctuations in relativistic heavy-ion collisions at top RHIC and LHC energies, Phys

    L. Du, Characterizing radial flow fluctuations in relativistic heavy-ion collisions at top RHIC and LHC energies, Phys. Rev. C 113 (1) (2026) 014901.arXiv:2508.07184

    2026

  7. [15]

    Zhu, X.-Y

    J. Zhu, X.-Y . Wu, G.-Y . Qin, Mapping Nuclear Deformation with Differential Radial Flow in Heavy-Ion Collisions (2 2026).arXiv: 2602.04148

    2026

  8. [16]

    STAR Collaboration, Engineering the shapes of quark-gluon plasma droplets by comparing anisotropic flow in small symmetric and asym- metric collision systems (10 2025).arXiv:2510.19645

    2025

  9. [17]

    Jia, Sources of radial flow fluctuations in the quark-gluon plasma, Phys

    J. Jia, Sources of radial flow fluctuations in the quark-gluon plasma, Phys. Rev. Lett. 136 (2026) 112301.arXiv:2507.14399. 4

    2026

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