Deciphering the universal scaling of particle transverse momentum spectra in heavy-ion collisions
Pith reviewed 2026-05-10 05:57 UTC · model grok-4.3
The pith
Universal scaling appears in particle transverse momentum spectra when divided by multiplicity and mean pT, explained by the Cooper-Frye formula and matching Hwa-Yang scaling.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Transverse momentum spectra scaled by the product of total multiplicity and mean transverse momentum exhibit a universal functional form across a wide range of collision energies and centralities in heavy-ion data, arising naturally from the Cooper-Frye hadronization in hydrodynamics and being identical to the Hwa-Yang scaling.
What carries the argument
The Cooper-Frye formula, which computes the particle yield from the thermal distribution on the freeze-out hypersurface in hydrodynamic simulations, allowing the spectra to be expressed as a universal function modulated by global multiplicity and mean pT.
If this is right
- The scaling should be valid for other hadron species under similar conditions.
- Breakdown occurs when non-hydrodynamic processes like jet fragmentation become important at high pT.
- The equivalence links recent experimental observations to established theoretical scaling from the early 2000s.
Where Pith is reading between the lines
- If the Cooper-Frye mechanism is responsible, the scaling could serve as a diagnostic for the applicability of hydrodynamics in a given collision system.
- Similar scaling might be testable in high-multiplicity proton-proton collisions if they exhibit collective behavior.
- Using the scaled spectra could help constrain the freeze-out temperature and flow velocity parameters more effectively.
Load-bearing premise
The universal scaling is caused by the Cooper-Frye formula within a hydrodynamic description of the collision rather than by other dynamical mechanisms or biases in the data selection.
What would settle it
A clear failure of the scaled spectra to collapse onto a single curve in central collisions at low to moderate pT, or the appearance of the scaling in systems where hydrodynamic evolution is not expected.
Figures
read the original abstract
We systematically investigate the scaling properties of the transverse momentum spectra for pions, kaons, and protons in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV, as well as in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV, across different centrality classes, using experimental data from the collaborations at the Relativistic Heavy Ion Collider (RHIC). Universal scaling emerges when the particle transverse momentum spectra are scaled by global physical quantities, i.e., the average total particle multiplicity and mean transverse momentum, confirming recent scaling findings from the data at the Large Hadron Collider (LHC) by the ExTrEMe collaboration. The scaling behavior breaks down in the high $p_{T}$ region and in peripheral collisions. We provide a natural explanation for these observations by invoking the Cooper-Frye formula, which is used for hadronization in hydrodynamics. Furthermore, we demonstrate the equivalence between the scaling found by the ExTrEMe collaboration and the Hwa-Yang scaling which was proposed two decades ago.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript systematically analyzes transverse momentum spectra of pions, kaons, and protons in Au+Au collisions at RHIC energies 7.7–200 GeV and U+U at 193 GeV across centralities using published data. It reports that scaling the spectra by average total particle multiplicity and mean p_T produces a universal collapse, confirming LHC ExTrEMe results, with breakdowns at high p_T and in peripheral collisions. The authors invoke the Cooper-Frye formula for a hydrodynamic explanation and algebraically demonstrate equivalence to the two-decade-old Hwa-Yang scaling.
Significance. If the data collapse is robust, the work provides a compact, parameter-free characterization of spectra across energies and systems, linking empirical observations to hydrodynamics and reviving Hwa-Yang scaling. The strength lies in the broad, systematic comparison of RHIC datasets; the hydrodynamic interpretation, however, remains underdeveloped relative to the empirical claim.
major comments (2)
- [Abstract and hydrodynamic discussion] Abstract and discussion of hydrodynamic explanation: the assertion that the Cooper-Frye formula naturally yields the observed scaling lacks an explicit reduction. The integral form dN/dp_T ∝ ∫ (p·u) exp(−p·u/T) dΣ does not automatically become a universal function of p_T / <p_T> after normalization by total multiplicity (volume factor) and <p_T> (T- and flow-dependent) without additional assumptions on constant T, boost invariance, or hypersurface details; these steps are not performed in the manuscript.
- [Equivalence to Hwa-Yang scaling] Section demonstrating equivalence to Hwa-Yang scaling: while the algebraic equivalence is stated, the manuscript should specify whether the equivalence follows from the same Cooper-Frye assumptions used for the hydrodynamic explanation or is a purely formal identity independent of the underlying dynamics.
minor comments (1)
- [Data analysis and figures] The manuscript would benefit from explicit statements of how uncertainties are propagated through the scaling procedure and from tabulated centrality bin definitions for each energy.
Simulated Author's Rebuttal
We thank the referee for the careful review and constructive feedback on our manuscript. We address each major comment below and have revised the manuscript to strengthen the presentation.
read point-by-point responses
-
Referee: [Abstract and hydrodynamic discussion] Abstract and discussion of hydrodynamic explanation: the assertion that the Cooper-Frye formula naturally yields the observed scaling lacks an explicit reduction. The integral form dN/dp_T ∝ ∫ (p·u) exp(−p·u/T) dΣ does not automatically become a universal function of p_T / <p_T> after normalization by total multiplicity (volume factor) and <p_T> (T- and flow-dependent) without additional assumptions on constant T, boost invariance, or hypersurface details; these steps are not performed in the manuscript.
Authors: We agree that an explicit reduction from the Cooper-Frye integral to the observed scaling form would improve clarity. In the revised manuscript we add an appendix that performs this derivation under the standard assumptions of boost-invariant hydrodynamics with constant freeze-out temperature and a simple cylindrical hypersurface. The steps show how normalization by total multiplicity (volume factor) and mean p_T (which absorbs the combined effects of T and collective flow) yields an approximate universal function of p_T/<p_T> in the hydrodynamic regime, while the observed breakdowns at high p_T and in peripheral collisions remain outside this regime. revision: yes
-
Referee: [Equivalence to Hwa-Yang scaling] Section demonstrating equivalence to Hwa-Yang scaling: while the algebraic equivalence is stated, the manuscript should specify whether the equivalence follows from the same Cooper-Frye assumptions used for the hydrodynamic explanation or is a purely formal identity independent of the underlying dynamics.
Authors: The algebraic equivalence between the ExTrEMe scaling and Hwa-Yang scaling is a formal mathematical identity obtained by re-expressing the scaling variables; it does not depend on the Cooper-Frye formula or any hydrodynamic assumptions. We will revise the relevant section to state this explicitly, noting that the physical motivation for the scaling can be linked to hydrodynamics via Cooper-Frye, while the equivalence itself is independent of the underlying dynamics. revision: yes
Circularity Check
No significant circularity: data collapse and algebraic equivalence are independent of fitted inputs
full rationale
The paper reports an empirical observation of data collapse when transverse momentum spectra are normalized by measured global quantities (average multiplicity and mean pT) extracted directly from the same RHIC datasets. This normalization is the standard procedure for testing scaling and does not involve fitting model parameters that are then relabeled as predictions. The claimed equivalence to Hwa-Yang scaling is described as an algebraic demonstration, not a reduction to self-citation or ansatz. Invocation of the Cooper-Frye formula is presented as a qualitative explanation rather than a load-bearing derivation that forces the observed scaling by construction. No self-citation chains, self-definitional loops, or fitted-input-as-prediction patterns appear in the central claims. The result remains falsifiable against independent datasets and does not reduce to its own inputs.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The Cooper-Frye formula provides the correct mapping from hydrodynamic fluid elements to observed particle spectra at freeze-out.
Reference graph
Works this paper leans on
-
[1]
R. C. Hwa and X. N. Wang, eds.,Quark-gluon plasma 3 (2004)
work page 2004
-
[2]
R. C. Hwa and X.-N. Wang, eds.,Quark-gluon plasma 4 (2010)
work page 2010
-
[3]
L. P. Csernai,Introduction to Relativistic Heavy Ion Col- lisions(Wiley, New York, 1994)
work page 1994
-
[4]
C. Y. Wong,Introduction to High-Energy Heavy-Ion Col- lisions(World Scientific, Singapore, 1994)
work page 1994
-
[5]
J. Chenet al., Properties of the QCD matter: review of selected results from the relativistic heavy ion collider beam energy scan (RHIC BES) program, Nucl. Sci. Tech. 35, 214 (2024)
work page 2024
-
[6]
X. He, S. Shi, and N. Xu, Physics of collec- tivity and EOS from the RHIC Beam Energy Scan program, Eur. Phys. J. Spec. Top. (2026), https://doi.org/10.1140/epjs/s11734-026-02261-7
-
[7]
Y. Zhang, Z. Wang, X. Luo, and N. Xu, Search for the QCD critical point in high energy nuclear colli- sions: a status report, Eur. Phys. J. Spec. Top. (2026), https://doi.org/10.1140/epjs/s11734-026-02226-w
-
[8]
Mohapatra, Experimental overview on flow observ- ables in heavy ion collisions, Nucl
S. Mohapatra, Experimental overview on flow observ- ables in heavy ion collisions, Nucl. Phys. A956, 59 (2016)
work page 2016
-
[9]
J. Adamet al.(ALICE Collaboration), Centrality evolu- tion of the charged–particle pseudorapidity density over a broad pseudorapidity range in Pb–Pb collisions at√sNN = 2.76 TeV, Phys. Lett. B754, 373 (2016)
work page 2016
-
[10]
B. I. Abelevet al.(STAR Collaboration), Rapidity and 10 species dependence of particle production at large trans- verse momentum for d+Au collisions at √sN N = 200- GeV, Phys. Rev. C76, 054903 (2007)
work page 2007
-
[11]
J. Q. Tao, M. Wang, H. Zheng, W. C. Zhang, L. L. Zhu, and A. Bonasera, Pseudorapidity distributions of charged particles in pp(¯p), p(d)A and AA collisions using Tsallis thermodynamics, J. Phys. G48, 105102 (2021)
work page 2021
-
[12]
J. Tao, H. He, H. Zheng, W. Zhang, X. Liu, L. Zhu, and A. Bonasera, Pseudo-rapidity distributions of charged particles in asymmetric collisions using Tsallis thermo- dynamics, Nucl. Sci. Tech.34, 172 (2023)
work page 2023
-
[13]
W. Zhao, L. Zhu, H. Zheng, C. M. Ko, and H. Song, Spectra and flow of light nuclei in relativistic heavy ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Col- lider, Phys. Rev. C98, 054905 (2018)
work page 2018
- [14]
-
[15]
S. Jena and R. Gupta, A unified formalism to study transverse momentum spectra in heavy-ion collision, Phys. Lett. B807, 135551 (2020)
work page 2020
-
[16]
R. J. Fries, B. Muller, C. Nonaka, and S. A. Bass, Hadronization in heavy ion collisions: Recombination and fragmentation of partons, Phys. Rev. Lett.90, 202303 (2003)
work page 2003
-
[17]
J. Adamset al.(STAR Collaboration), Multistrange baryon production in Au-Au collisions at √sN N = 130 GeV, Phys. Rev. Lett.92, 182301 (2004)
work page 2004
- [18]
-
[19]
J. Tao, W. Wu, M. Wang, H. Zheng, W. Zhang, L. Zhu, and A. Bonasera, The Novel Scaling of Tsallis Parame- ters from the Transverse Momentum Spectra of Charged Particles in Heavy-Ion Collisions, Particles5, 146 (2022)
work page 2022
- [20]
-
[21]
M. A. C. Lamont, Investigation of the high-p T strange baryon anomalies at RHIC, Eur. Phys. J. C49, 35 (2007)
work page 2007
-
[22]
A. Adareet al.(PHENIX Collaboration), Spectra and ratios of identified particles in Au+Au andd+Au col- lisions at √sN N = 200 GeV, Phys. Rev. C88, 024906 (2013)
work page 2013
-
[23]
B. B. Abelevet al.(ALICE Collaboration), Production of charged pions, kaons and protons at large transverse momenta in pp and Pb–Pb collisions at√sNN =2.76 TeV, Phys. Lett. B736, 196 (2014)
work page 2014
-
[24]
K. Adcoxet al.(PHENIX Collaboration), Suppression of hadrons with large transverse momentum in central Au+Au collisions at √sN N = 130-GeV, Phys. Rev. Lett. 88, 022301 (2002)
work page 2002
-
[25]
J. Adamset al.(STAR Collaboration), Evidence from d + Au measurements for final state suppression of highp T hadrons in Au+Au collisions at RHIC, Phys. Rev. Lett. 91, 072304 (2003)
work page 2003
-
[26]
Csanadet al., Universal scaling of the elliptic flow data at RHIC, Eur
M. Csanadet al., Universal scaling of the elliptic flow data at RHIC, Eur. Phys. J. A38, 363 (2008)
work page 2008
-
[27]
M. Wang, J.-Q. Tao, H. Zheng, W.-C. Zhang, L.-L. Zhu, and A. Bonasera, Number-of-constituent-quark scaling of elliptic flow: a quantitative study, Nucl. Sci. Tech.33, 37 (2022)
work page 2022
-
[28]
L. Adamczyket al.(STAR Collaboration), Centrality and transverse momentum dependence of elliptic flow of multistrange hadrons andϕmeson in Au+Au colli- sions at √sN N = 200 GeV, Phys. Rev. Lett.116, 062301 (2016)
work page 2016
-
[29]
Adamset al.(STAR Collaboration), Azimuthal anisotropy in Au+Au collisions at √sN N = 200-GeV, Phys
J. Adamset al.(STAR Collaboration), Azimuthal anisotropy in Au+Au collisions at √sN N = 200-GeV, Phys. Rev. C72, 014904 (2005)
work page 2005
-
[30]
C. Adleret al.(STAR Collaboration), Elliptic flow from two and four particle correlations in Au+Au collisions at√sN N = 130-GeV, Phys. Rev. C66, 034904 (2002)
work page 2002
-
[31]
Z. Koba, H. B. Nielsen, and P. Olesen, Scaling of mul- tiplicity distributions in high-energy hadron collisions, Nucl. Phys. B40, 317 (1972)
work page 1972
-
[32]
Sarcevic, KNO scaling in hadron-hadron collisions, Acta Phys
I. Sarcevic, KNO scaling in hadron-hadron collisions, Acta Phys. Polon. B19, 361 (1988)
work page 1988
-
[33]
X.-P. Duan, L. Chen, G.-L. Ma, C. A. Salgado, and B. Wu, Koba-Nielsen-Olesen scaling in quark and gluon jets at the LHC, Phys. Rev. D112, 094022 (2025)
work page 2025
-
[34]
J. D. Bjorken, Asymptotic Sum Rules at Infinite Momen- tum, Phys. Rev.179, 1547 (1969)
work page 1969
-
[35]
G. Altarelli and G. Parisi, Asymptotic Freedom in Parton Language, Nucl. Phys. B126, 298 (1977)
work page 1977
-
[36]
Regge, Introduction to complex orbital momenta, Nuovo Cim.14, 951 (1959)
T. Regge, Introduction to complex orbital momenta, Nuovo Cim.14, 951 (1959)
work page 1959
-
[37]
A. R. White, Analytic multi - Regge theory and the pomeron in QCD. 2. Gauge theory analysis, Int. J. Mod. Phys. A8, 4755 (1993)
work page 1993
-
[38]
M. A. Braun, E. M. Kuzminskii, and M. I. Vyazovsky, On the reggeon model with the pomeron and odderon: singularities with non-zero masses, Eur. Phys. J. C85, 1415 (2025)
work page 2025
-
[39]
R. P. Feynman, Very high-energy collisions of hadrons, Phys. Rev. Lett.23, 1415 (1969)
work page 1969
-
[40]
J. Benecke, T. T. Chou, C.-N. Yang, and E. Yen, Hypoth- esis of Limiting Fragmentation in High-Energy Collisions, Phys. Rev.188, 2159 (1969)
work page 1969
-
[41]
G. H. Arakelyan, C. Merino, C. Pajares, and Y. M. Sha- belski, Feynman scaling violation on baryon spectra in pp collisions at LHC and cosmic ray energies, Phys. Atom. Nucl.76, 316 (2013)
work page 2013
- [42]
-
[43]
R. C. Hwa and C. B. Yang, Scaling behavior at highp T and thep/πratio, Phys. Rev. C67, 034902 (2003)
work page 2003
-
[44]
R. C. Hwa and C. B. Yang, Centrality scaling of thep T distribution of pions, Phys. Rev. Lett.90, 212301 (2003)
work page 2003
-
[45]
R. C. Hwa and C. B. Yang, Scaling distributions of quarks, mesons and proton for allp T, energy and cen- trality, Phys. Rev. C67, 064902 (2003)
work page 2003
-
[46]
L. L. Zhu and C. B. Yang, Universal scaling ofp T distri- bution of particles in relativistic nuclear collisions, Phys. Rev. C75, 044904 (2007)
work page 2007
-
[47]
F.-L. Zheng and C.-B. Yang, Scalingp T distribution in small rapidity region and pseudorapidity asymmetry for charged hadron in d + Au collisions at√sN N = 200-GeV, Commun. Theor. Phys.49, 1580 (2008)
work page 2008
-
[48]
L. L. Zhu, H. Zheng, and C. B. Yang, Scaling and mul- tiplicity dependence ofp T spectra in p p collisions at√sN N = 200-GeV, Nucl. Phys. A809, 259 (2008)
work page 2008
-
[49]
L. L. Zhu, H. Zheng, and C. B. Yang, Scaling Behavior of Transverse Kinetic Energy Distributions in Au+Au Collisions at √sN N = 200-GeV, Nucl. Phys. A802, 122 (2008). 11
work page 2008
-
[50]
W. C. Zhang, Y. Zeng, W. X. Nie, L. L. Zhu, and C. B. Yang, Scalingp T distributions forpand ¯pproduced in Au + Au collisions at RHIC, Phys. Rev. C76, 044910 (2007)
work page 2007
-
[51]
W. C. Zhang and C. B. Yang, Scaling behavior of charged hadronp T distributions inppandp¯pcollisions, J. Phys. G41, 105006 (2014)
work page 2014
-
[52]
W. C. Zhang, Scaling behaviours of thep T spectra for identified hadrons inppcollisions, J. Phys. G43, 015003 (2016)
work page 2016
-
[53]
L. Yang, Y. Wang, W. Hao, N. Liu, X. Du, and W. Zhang, Universal scaling of strange particle pT spectra in pp col- lisions, Eur. Phys. J. A54, 54 (2018)
work page 2018
-
[54]
Y. Wang, L. Yang, X. Du, N. Liu, L. Qiao, and W. Zhang, Universal scaling of the pion, kaon and protonp T spectra in Pb-Pb collisions at 2.76 TeV, Nucl. Phys. A976, 46 (2018)
work page 2018
-
[55]
N. Liu, X. Du, L. Qiao, G. Che, and W. Zhang, Universal scaling of meson and baryon spectra in p-Pb collisions at 5.02TeV, Int. J. Mod. Phys. E28, 1950030 (2019)
work page 2019
-
[56]
C. D. Muncinelli, F. G. Gardim, D. D. Chinellato, G. S. Denicol, A. V. Giannini, M. Luzum, J. Noronha, T. N. da Silva, J. Takahashi, and G. Torrieri (ExTrEMe Col- laboration), Universality of scaled particle spectra in ul- trarelativistic heavy-ion collisions, Phys. Rev. C112, 064922 (2025)
work page 2025
- [57]
- [58]
- [59]
-
[60]
L. Adamczyket al.(STAR Collaboration), Bulk Proper- ties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program, Phys. Rev. C96, 044904 (2017)
work page 2017
-
[61]
J. Adamet al.(STAR Collaboration), Bulk properties of the system formed inAu+Aucollisions at √sNN =14.5 GeV at the BNL STAR detector, Phys. Rev. C 101, 024905 (2020)
work page 2020
-
[62]
B. I. Abelevet al.(STAR Collaboration), Energy depen- dence ofπ ±,pand ¯ptransverse momentum spectra for Au+Au collisions at √sN N = 62.4 and 200-GeV, Phys. Lett. B655, 104 (2007)
work page 2007
-
[63]
S. S. Adleret al.(PHENIX Collaboration), Identified charged particle spectra and yields in Au+Au collisions at √sN N = 200-GeV, Phys. Rev. C69, 034909 (2004)
work page 2004
-
[64]
M. S. Abdallahet al.(STAR Collaboration), Pion, kaon, and (anti)proton production in U+U collisions at √sN N =193 GeV measured with the STAR detector, Phys. Rev. C107, 024901 (2023)
work page 2023
- [65]
-
[66]
C. Gale, S. Jeon, and B. Schenke, Hydrodynamic Mod- eling of Heavy-Ion Collisions, Int. J. Mod. Phys. A28, 1340011 (2013)
work page 2013
-
[67]
W. Florkowski, M. P. Heller, and M. Spalinski, New the- ories of relativistic hydrodynamics in the LHC era, Rept. Prog. Phys.81, 046001 (2018)
work page 2018
-
[68]
B. I. Abelevet al.(STAR Collaboration), Centrality de- pendence of charged hadron and strange hadron elliptic flow from √sN N = 200-GeV Au + Au collisions, Phys. Rev. C77, 054901 (2008)
work page 2008
- [69]
-
[70]
L.-G. Pang, H. Petersen, and X.-N. Wang, Pseudo- rapidity distribution and decorrelation of anisotropic flow within the open-computing-language implementa- tion CLVisc hydrodynamics, Phys. Rev. C97, 064918 (2018)
work page 2018
-
[71]
F. Cooper and G. Frye, Comment on the Single Particle Distribution in the Hydrodynamic and Statistical Ther- modynamic Models of Multiparticle Production, Phys. Rev. D10, 186 (1974)
work page 1974
-
[72]
F. Becattini, V. Chandra, L. Del Zanna, and E. Grossi, Relativistic distribution function for particles with spin at local thermodynamical equilibrium, Annals Phys.338, 32 (2013)
work page 2013
-
[73]
K. Urmossy and T. S. Biro, Cooper-Frye Formula and Non-extensive Coalescence at RHIC Energy, Phys. Lett. B689, 14 (2010)
work page 2010
-
[74]
G. Che, J. Gu, W. Zhang, and H. Zheng, Identified parti- cle spectra in Pb–Pb, Xe–Xe and p–Pb collisions with the Tsallis blast-wave model, J. Phys. G48, 095103 (2021)
work page 2021
-
[75]
F. Retiere and M. A. Lisa, Observable implications of geometrical and dynamical aspects of freeze out in heavy ion collisions, Phys. Rev. C70, 044907 (2004)
work page 2004
-
[76]
W. Florkowski and W. Broniowski, Hydro-inspired pa- rameterizations of freeze-out in relativistic heavy-ion col- lisions, Acta Phys. Polon. B35, 2895 (2004)
work page 2004
-
[77]
E. Schnedermann, J. Sollfrank, and U. W. Heinz, Ther- mal phenomenology of hadrons from 200-A/GeV S+S collisions, Phys. Rev. C48, 2462 (1993)
work page 1993
-
[78]
B. Abelevet al.(ALICE Collaboration), Centrality de- termination of Pb-Pb collisions at √sN N = 2.76 TeV with ALICE, Phys. Rev. C88, 044909 (2013)
work page 2013
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.