REVIEW 4 major objections 5 minor 80 references
Constraining the chiral magnetic effect using spectator and participant planes across Au+Au and isobar collisions at $\sqrt{s_{_{\rm NN}}} = 200$ GeV
T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read The paper claims that the two-plane CME extraction is more reliable in Au+Au than in isobar collisions, with $b/a = 0.88 \pm 0.08$, and that Au+Au shows a stronger CME signal when the spectator plane is used.
desk verdict The b/a=0.88 result for Au+Au is a credible model output, but the paper's central claim of a stronger CME in Au+Au rests on a chi-square analysis that is internally inconsistent across the two planes and statistically fragile. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the two-plane decomposition of the charge-dependent azimuthal correlator $\Delta\gamma\{\psi\} = \Delta\gamma_{\rm Bkg}\{\psi\} + \Delta\gamma_{\rm CME}\{\psi\}$, where $\psi$ is either the spectator plane $\psi_{\rm SP}$ or the participant plane $\psi_{\rm PP}$. The ratios $a = v_2\{SP\}/v_2\{PP\}$ and $A = \Delta\gamma\{SP\}/\Delta\gamma\{PP\}$ enter the standard extraction formula $f_{\rm CME} = (A/a - 1)/(1/a^2 - 1)$, which assumes the CME signal ratio is the same as the flow ratio; the paper replaces this with $b = \Delta\gamma_{\rm CME}\{PP\}/\Delta\gamma_{\rm CME}\{SP\}$, obtained from Eq. (13) as the difference between CME-on and CME-off AMPT simulations. The key mechanism is that final-state rescattering rotates and damps the CME current, decorrelating it from the participant plane while the spectator plane keeps tracking the magnetic field direction; this decorrelation is weaker in Au+Au than in Ru+Ru, which is why $b/a$ stays closer to 1 in Au+Au.
What would settle it
In STAR data for 20–50% centrality, the spectator-plane double ratio $(\Delta\gamma/v_2)\,dN_{\rm ch}/d\eta$ should order Au+Au > Ru+Ru > Zr+Zr if the paper's chi-square conclusion is right; a measurement with small enough errors that breaks this ordering would falsify the claim of a stronger CME in Au+Au.
Extended reading notes
Core claim
The paper's central claim is that the ratio $b = \Delta\gamma_{\rm CME}\{PP\}/\Delta\gamma_{\rm CME}\{SP\}$, computed in the AMPT model by subtracting the no-CME ($p=0$) simulation from the CME-on ($p>0$) simulation via Eq. (13), is not equal to $a = v_2\{SP\}/v_2\{PP\}$, and that the discrepancy is smaller in Au+Au than in isobar collisions. In 20–50% centrality Au+Au collisions the paper finds $b/a = 0.88 \pm 0.08$, whereas its earlier isobar study found $0.65 \pm 0.18$. Because the modified two-plane formula $f_{\rm CME} = (A/a - 1)/(1/ab - 1)$ reduces to the standard formula only when $b=a$, the Au+Au value shows the correction is mild there. Stage-by-stage evolution in the AMPT model shows $b$ falling as final-state interactions decorrelate the CME current from the participant plane, but less steeply in Au+Au than in Ru+Ru. A simultaneous chi-square fit to STAR data for the three systems prefers a CME strength near 2% when the participant plane is used, but 7.5–10% for Au+Au (5–7.5% for isobars) when the spectator plane is used, which the authors read as a stronger CME signature in Au+Au.
Load-bearing premise
The central conclusion depends on the AMPT model correctly capturing how final-state rescattering rotates and damps the chiral-magnetic-effect current relative to the participant plane; if that decorrelation is unrealistic, the $b/a = 0.88$ value and the Au+Au advantage would not transfer to experiment.
Editorial extensions
If this is right
- Two-plane extractions of the CME fraction should quote $f_{\rm CME}\{b\}$ using a transport-model value of $b$, rather than assuming $b=a$; in Au+Au the correction is mild, so the standard formula is a better approximation there than in isobars.
- The spectator-plane $\Delta\gamma$ is the observable to focus on: the model fits require a CME strength of 7.5–10% in Au+Au versus about 2% for the participant plane, so a positive CME signal of that size should be visible in STAR data.
- The stage-by-stage decrease of $b/a$ in the AMPT model means the two-plane method's reliability is system-size dependent, not a fixed property of the observable.
- If Au+Au indeed has a stronger CME signature than Ru+Ru and Zr+Zr, then the isobar upper limits do not directly constrain the CME in Au+Au.
- The double-ratio $\Delta\gamma/v_2 \, dN_{\rm ch}/d\eta$ with respect to the spectator plane is the discriminator that separates Au+Au from the isobar systems in the chi-square analysis.
Reading between the lines
- If the trend $b/a \to 1$ with increasing system size is real, then the two-plane method should be even more reliable in larger collision systems such as U+U or Pb+Pb, and less reliable in smaller systems; this extrapolation is not made in the paper.
- The gap between the participant-plane fit ($p \approx 2\%$) and the spectator-plane fit ($p \approx 7.5$–$10\%$) suggests the CME signal may be partly hidden in the participant-plane view; a measurement constraining both planes with better non-flow control would decide which plane carries the truth.
- A natural independent test would be to compute $b$ in a different framework, such as anomalous hydrodynamics with a time-dependent magnetic field; agreement with $b/a=0.88$ for Au+Au and $0.65$ for isobars would confirm that the AMPT decorrelation mechanism is the right physics.
- Because the paper uses the initial-state spectator plane as a proxy for the magnetic field direction, a direct reconstruction of the field direction (or a calculation with a fully dynamical field) could either strengthen or weaken the claimed spectator-plane advantage.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript extends the authors' previous AMPT-based two-plane study of the chiral magnetic effect (CME) from isobar collisions to Au+Au collisions at sqrt(s_NN)=200 GeV. It implements a CME-like charge separation with tunable strength p, reconstructs spectator and participant planes, and computes the ratio b = Delta-gamma_CME{PP}/Delta-gamma_CME{SP} through Eq. (13). The authors report b/a = 0.88 +/- 0.08 for Au+Au, compare it with their earlier isobar value 0.65 +/- 0.18, and perform a chi-square scan over p for Au+Au, Ru+Ru, and Zr+Zr using the double ratio Delta-gamma/(v2 dNch/deta). On this basis they claim that the two-plane method is less biased in Au+Au collisions and that Au+Au exhibits a stronger CME signal, especially with respect to the spectator plane.
Significance. The algebraic derivation of fCME{b} in Eq. (12) is correct, and the systematic use of AMPT to track a and b through the evolution stages is informative; the paper offers a falsifiable, quantitative framework for interpreting two-plane measurements. If the claimed b/a contrast and the chi-square hierarchy were statistically robust, the result would be valuable for planning Au+Au measurements. However, the central cross-system conclusion currently rests on a marginally significant b/a difference and on chi-square fits that use only model errors and a coarse p grid; the internal inconsistency between participant-plane and spectator-plane fits prevents the data from establishing the stronger-CME claim. The paper is a useful model study, but its headline conclusion is not yet supported at the claimed level.
major comments (4)
- [Sec. III, Fig. 11 and Sec. IV] The headline comparison b/a(Au+Au) = 0.88 +/- 0.08 versus b/a(isobar) = 0.65 +/- 0.18 is not statistically significant: the difference is roughly 0.23 while the quadrature-summed uncertainty is about 0.20, corresponding to about 1.2 sigma. Statements in the abstract and in Sec. IV that Au+Au shows a 'reduced difference' and 'enhances the experimental reliability' therefore go beyond what the two model outputs establish. Please provide a proper propagation of the two determinations, or soften the claim to a suggestive trend.
- [Sec. III, Eqs. (14)-(15) and Fig. 17] The chi-square definition in Eq. (14) uses only the model error w_o in the denominator and omits the experimental uncertainties on E_i, and the p grid is coarse (0, 2, 5, 7.5, 10%) with no confidence intervals or Delta-chi^2 thresholds. The reported differences between best-fit p values, for example Au+Au at 7.5% versus Ru+Ru at 5% and Au+Au at 10% versus Zr+Zr at 7.5%, are therefore not shown to be significant. A combined fit that includes experimental errors and reports Delta-chi^2 contours is needed before these values can support the claim of stronger CME in Au+Au.
- [Sec. III, Fig. 17 panels (a)-(d)] The participant-plane fits select p about 2% for all three systems, while the spectator-plane fits select p = 7.5-10% for Au+Au and p = 5-7.5% for the isobars. The text interprets this as greater SP sensitivity, but no combined fit is performed. If the PP observable is insensitive to the CME, the PP fit cannot constrain p and cannot be used to argue that the CME is small; if it is sensitive, the two planes give mutually inconsistent CME strengths for the same systems. Either way, the present analysis does not establish that Au+Au has a stronger CME signal.
- [Sec. II C, Eqs. (12)-(13) and Figs. 9-11] The value b/a = 0.88 +/- 0.08 is an internal AMPT output, computed from the same model used to generate the CME signal, and the extraction procedure is underspecified: the text does not state which p values, centrality bins, and error treatment enter the constant-fit value, and the p = 2% point is excluded 'due to its large statistical uncertainties' without a documented criterion. Because the claimed Au+Au advantage is driven by the stage-dependent decrease of b/a in Figs. 15-16, the model uncertainty on b/a (for example, from parton cross-section or string-melting parameters) should be quantified for the conclusion to be robust.
minor comments (5)
- [Eq. (14)] The sum is written as 'nX k=0' and the indices k and n are inconsistent; please define all indices and clarify what 'normalized chi-square' means.
- [Fig. 14] The quantity fCME{p} is plotted but never defined in the text; please add its definition or cite the precise equation from Ref. [64].
- [Eqs. (2)-(5)] The notation <...> is used both for event averages in Eqs. (2)-(3) and in the double-average notation of Eqs. (4)-(5); please define the averaging convention explicitly.
- [Introduction and Fig. 6 caption] There are several typographical issues, including the stray Chinese comma after 'AMPT model ,' in the introduction and inconsistent comma usage in the Fig. 6 caption.
- [References] Ref. [57] is an arXiv preprint from 2024; please update it if a published version now exists.
Circularity Check
No significant circularity: the b correction is a model output and the Au+Au conclusion is a fit interpretation, not a circular reduction; the isobar baseline is self-cited but anchored to published STAR comparisons.
full rationale
After walking the derivation chain, I find no circular step that satisfies the hard evidence test. The quantity b is not fitted to the STAR observables it later corrects: Eq. (13) defines b as the ratio of AMPT Δγ differences between CME-on and CME-off (p≠0 vs p=0) runs, and the b/a=0.88 value is a constant fit to those model outputs, not to the experimental Δγ ratios. The p values in the Sec. III chi-square analysis (Eq. (14), Fig. 17) are tunable model inputs; the abstract's 'stronger CME in Au+Au' is a restatement of which grid point (Au+Au p=7.5-10% vs isobar p=5-7.5% in the SP panels) minimizes χ². This is a parameter constraint, not an independent prediction, so it may be statistically fragile but it is not circular. The isobar comparison is admittedly imported from the same authors' previous work ('all the calculations about Ru+Ru and Zr+Zr collisions are taken from Ref. [64]'), and the b/a=0.65 baseline is load-bearing; however, Ref. [64] is a published, externally STAR-data-anchored calculation rather than an unverified uniqueness theorem, so this self-citation does not make the derivation circular. I therefore assign score 2.
Assumptions & free parameters
free parameters (3)
- p_Au (CME strength in Au+Au) =
PP fit: ~2%; SP fit: 7.5-10%
- p_Ru (CME strength in Ru+Ru) =
PP fit: ~2%; SP fit: ~5%
- p_Zr (CME strength in Zr+Zr) =
PP fit: ~2%; SP fit: ~7.5%
assumptions (4)
- domain assumption Background Δγ is dominated by elliptic flow and scales as v2 relative to each plane.
- domain assumption The CME signal component satisfies ΔγCME{PP} = b ΔγCME{SP} with b independent of centrality within 20-50%.
- domain assumption AMPT's initial CME implementation from Ref. [75] adequately represents the real CME current and its decorrelation during evolution.
- standard math The ratio a equals <cos 2(ψPP - ψSP)>.
Cite this review
Pith. "Pith review of Constraining the chiral magnetic effect using spectator and participant planes across Au+Au and isobar collisions at $\sqrt{s_{_{\rm NN}}} = 200$ GeV." pith.science (2026). https://pith.science/paper/VNBNDRIF
@misc{pith2026250102794,
author = {Pith},
title = {Pith review of: Constraining the chiral magnetic effect using spectator and participant planes across Au+Au and isobar collisions at $\sqrts__\rm NN = 200$ GeV},
year = {2026},
howpublished = {\url{https://pith.science/paper/VNBNDRIF}},
note = {Machine review of arXiv:2501.02794}
}
abstract
We investigate the chiral magnetic effect (CME) in relativistic heavy-ion collisions through an improved two-plane method analysis of the $\Delta\gamma$ observable, probing $\mathcal{CP}$-symmetry breaking in strong interactions and topological properties of the QCD vacuum. Using a multiphase transport model with tunable CME strengths, we systematically compare Au+Au and isobar collisions at $\sqrt{s_{_{\rm NN}}} = 200$ GeV. We observe a reduced difference in the CME signal-to-background ratio between the spectator and participant planes for Au+Au collisions compared to isobar collisions. A comprehensive chi-square analysis across all three collision systems reveals stronger CME signatures in Au+Au collisions relative to isobar collisions, particularly when measured with respect to the spectator plane. Our findings demonstrate an enhanced experimental reliability of the two-plane method for the CME detection in Au+Au collisions.
Figures
Figures from the paper (14 more)
Reference graph
Works this paper leans on
-
[1]
P. F. Kolb, J. Sollfrank, and U. W. Heinz, Phys. Rev. C 62, 054909 (2000), hep-ph/0006129
arXiv 2000
- [2]
- [3]
- [4]
-
[5]
H. Song, Y. Zhou, and K. Gajdosova, Nucl. Sci. Tech. 28, 99 (2017), 1703.00670
arXiv 2017
- [6]
-
[7]
S. Wu, C. Shen, and H. Song, Chin. Phys. Lett. 38, 081201 (2021), 2104.13250
arXiv 2021
-
[8]
Properties of the QCD Matter: A Review of Selected Results from the ALICE Experiment
Q.-Y. Shou et al., Nucl. Sci. Tech. 35, 219 (2024), 2409.17964
work page Pith review arXiv 2024
Show all 80 references
-
[9]
Skokov, A
V. Skokov, A. Y. Illarionov, and V. Toneev, Int. J. Mod. Phys. A 24, 5925 (2009), 0907.1396
2009 arXiv
- [10]
- [11]
-
[12]
Zhao, Y.-G
X.-L. Zhao, Y.-G. Ma, and G.-L. Ma, Phys. Rev. C 97, 024910 (2018), 1709.05962
2018 arXiv
- [13]
-
[14]
Chen, X.-L
Y. Chen, X.-L. Sheng, and G.-L. Ma, Nucl. Phys. A 1011, 122199 (2021), 2101.09845
2021 arXiv
- [15]
-
[16]
Zhao, J.-H
J. Zhao, J.-H. Chen, X.-G. Huang, and Y.-G. Ma, Nucl. Sci. Tech. 35, 20 (2024), 2211.03968
2024 arXiv
- [17]
-
[18]
D. E. Kharzeev, L. D. McLerran, and H. J. Warringa, Nucl. Phys. A 803, 227 (2008), 0711.0950
2008 arXiv
-
[19]
Fukushima, D
K. Fukushima, D. E. Kharzeev, and H. J. Warringa, Phys. Rev. D 78, 074033 (2008), 0808.3382
2008 arXiv
-
[20]
S. A. Voloshin, Phys. Rev. C 70, 057901 (2004), hep-ph/0406311
2004 arXiv
-
[21]
B. I. Abelev et al. (STAR), Phys. Rev. Lett. 103, 251601 (2009), 0909.1739
2009 arXiv
-
[22]
B. I. Abelev et al. (STAR), Phys. Rev. C 81, 054908 (2010), 0909.1717
2010 arXiv
- [23]
- [24]
- [25]
- [26]
-
[27]
J. Liao, V. Koch, and A. Bzdak, Phys. Rev. C 82, 054902 (2010), 1005.5380
2010 arXiv
- [28]
- [29]
- [30]
- [31]
-
[32]
Wang and J
F.-Q. Wang and J. Zhao, Nucl. Sci. Tech. 29, 179 (2018)
2018
- [33]
- [34]
- [35]
- [36]
-
[37]
Qi-Ye et al., Acta Phys
S. Qi-Ye et al., Acta Phys. Sin. 72, 112504 (2023)
2023
-
[38]
Z. Xu, B. Chan, G. Wang, A. Tang, and H. Z. Huang, Phys. Lett. B 848, 138367 (2024), 2307.14997
2024 arXiv
-
[39]
Y.-S. Zhao, L. Wang, K. Zhou, and X.-G. Huang, Phys. Rev. C 106, L051901 (2022), 2105.13761
2022 arXiv
-
[40]
S. A. Voloshin, Phys. Rev. Lett. 105, 172301 (2010), 1006.1020
2010 arXiv
-
[41]
Deng, X.-G
W.-T. Deng, X.-G. Huang, G.-L. Ma, and G. Wang, Phys. Rev. C 94, 041901 (2016), 1607.04697
2016 arXiv
-
[42]
M. I. Abdulhamid et al. (STAR), Phys. Rev. Res. 6, L032005 (2024), 2308.16846
2024
-
[43]
M. I. Abdulhamid et al. (STAR), Phys. Rev. C 110, 014905 (2024), 2310.13096
2024
- [44]
-
[45]
H.-j. Xu, H. Li, X. Wang, C. Shen, and F. Wang, Phys. Lett. B 819, 136453 (2021), 2103.05595
2021 arXiv
-
[46]
H.-j. Xu, W. Zhao, H. Li, Y. Zhou, L.-W. Chen, and F. Wang, Phys. Rev. C 108, L011902 (2023), 2111.14812
2023 arXiv
- [47]
- [48]
-
[49]
Giacalone, J
G. Giacalone, J. Jia, and C. Zhang, Phys. Rev. Lett. 127, 242301 (2021), 2105.01638
2021 arXiv
- [50]
- [51]
- [52]
-
[53]
Li, Y.-G
F. Li, Y.-G. Ma, S. Zhang, G.-L. Ma, Q. Shou, and Q.-Y. Shou, Phys. Rev. C 106, 014906 (2022), 2201.10994
2022 arXiv
- [54]
-
[55]
Wang, H.-j
J.-f. Wang, H.-j. Xu, and F.-Q. Wang, Nucl. Sci. Tech. 35, 108 (2024), 2305.17114
2024 arXiv
-
[56]
Z. Yuan, A. Huang, W.-H. Zhou, G.-L. Ma, and M. Huang, Phys. Rev. C 109, L031903 (2024), 2310.20194
2024 arXiv
-
[57]
Z. Yuan, A. Huang, G. Xie, W.-H. Zhou, G.-L. Ma, and M. Huang (2024), 2412.09130
2024 arXiv
-
[58]
H.-j. Xu, J. Zhao, X. Wang, H. Li, Z.-W. Lin, C. Shen, and F. Wang, Chin. Phys. C 42, 084103 (2018), 1710.07265
2018 arXiv
-
[59]
H.-J. Xu, X. Wang, H. Li, J. Zhao, Z.-W. Lin, C. Shen, and F. Wang, Phys. Rev. Lett. 121, 022301 (2018), 1710.03086
2018 arXiv
-
[60]
Zhao, G.-L
X.-L. Zhao, G.-L. Ma, and Y.-G. Ma, Phys. Rev. C 99, 034903 (2019), 1901.04151
2019 arXiv
-
[61]
M. S. Abdallah et al. (STAR), Phys. Rev. Lett. 128, 092301 (2022), 2106.09243
2022
-
[62]
Y. Feng, J. Zhao, H. Li, H.-j. Xu, and F. Wang, Phys. Rev. C 105, 024913 (2022), 2106.15595
2022 arXiv
-
[63]
S. Shi, Y. Jiang, E. Lilleskov, and J. Liao, Annals Phys. 394, 50 (2018), 1711.02496
2018 arXiv
-
[64]
Chen, X.-L
B.-X. Chen, X.-L. Zhao, and G.-L. Ma, Phys. Rev. C 109, 024909 (2024), 2301.12076
2024 arXiv
-
[65]
Z.-W. Lin, C. M. Ko, B.-A. Li, B. Zhang, and S. Pal, Phys. Rev. C 72, 064901 (2005), nucl-th/0411110
2005 arXiv
- [66]
- [67]
- [68]
-
[69]
J. D. Orjuela Koop, A. Adare, D. McGlinchey, and J. L. Nagle, Phys. Rev. C 92, 054903 (2015), 1501.06880
2015 arXiv
-
[70]
Ma and A
G.-L. Ma and A. Bzdak, Nucl. Phys. A 956, 745 (2016)
2016
- [71]
- [72]
- [73]
-
[74]
Chen, H.-S
Q. Chen, H.-S. Wang, and G.-L. Ma, Phys. Rev. C 107, 034910 (2023), 2210.17125
2023 arXiv
- [75]
-
[76]
Abdallah et al
M. Abdallah et al. (STAR), Phys. Rev. C 105, 014901 (2022), 2109.00131
2022
-
[77]
Bloczynski, X.-G
J. Bloczynski, X.-G. Huang, X. Zhang, and J. Liao, Phys. Lett. B 718, 1529 (2013), 1209.6594
2013 arXiv
-
[78]
S. Shi, H. Zhang, D. Hou, and J. Liao, Phys. Rev. Lett. 125, 242301 (2020), 1910.14010
2020 arXiv
-
[79]
Huang, M.-W
L. Huang, M.-W. Nie, and G.-L. Ma, Phys. Rev. C 101, 024916 (2020), 1906.11631
2020 arXiv
- [80]
Reviewed August 10, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.