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arxiv: 2604.19650 · v1 · submitted 2026-04-21 · ⚛️ nucl-th · nucl-ex

Recognition: unknown

Probing the neutron-skin thickness through J/psi photoproduction in ultra-peripheral collisions

Haoyuan Li , Lu-Meng Liu , Jinhui Chen , Yu-Gang Ma , Chunjian Zhang
Authors on Pith no claims yet

Pith reviewed 2026-05-10 00:36 UTC · model grok-4.3

classification ⚛️ nucl-th nucl-ex
keywords neutron skin thicknessJ/ψ photoproductionultra-peripheral collisionscoherent cross sectionincoherent cross sectioncolor glass condensatemomentum transfer spectranuclear gluon distribution
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The pith

The thickness of the neutron skin in lead nuclei imprints a measurable signature on the momentum-transfer spectra of J/ψ photoproduction in ultra-peripheral collisions.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines how neutron-skin thickness affects J/ψ photoproduction in ultra-peripheral Pb+Pb collisions. In the Color Glass Condensate framework, a thicker skin produces a smoother, more extended nuclear color-density profile. This suppresses the coherent cross section at large momentum transfers while enhancing the incoherent cross section through greater event-by-event fluctuations at the nuclear edge. The ratio of incoherent to coherent integrated cross sections stands out as a sensitive probe with smaller theoretical uncertainties. If borne out, the approach could constrain neutron-skin thickness and nuclear gluon distributions at the LHC and future electron-ion colliders.

Core claim

We find a clear imprint of the neutron skin on the |t| spectra: a larger neutron skin leads to a smoother and more extended color-density profile, suppressing the coherent cross section at large |t| while enhancing the incoherent cross section through increased event-by-event configurational fluctuations in the nuclear periphery. We further show that the ratio of incoherent to coherent integrated cross sections provides a particularly sensitive and robust observable, with reduced theoretical uncertainties. These results establish diffractive vector-meson photoproduction in ultra-peripheral collisions as a powerful tomographic tool to constrain the neutron-skin thickness and the transverse gl

What carries the argument

The |t|-dependent coherent and incoherent J/ψ photoproduction cross sections calculated in the Color Glass Condensate framework, where neutron-skin thickness controls the smoothness and fluctuations of the nuclear color-density profile.

If this is right

  • A larger neutron skin suppresses coherent J/ψ production at high |t|.
  • A larger neutron skin enhances incoherent J/ψ production through increased peripheral fluctuations.
  • The ratio of integrated incoherent to coherent cross sections varies sensitively with neutron-skin thickness while carrying smaller theoretical uncertainties.
  • Diffractive photoproduction in ultra-peripheral collisions can serve as a tomographic probe of both neutron-skin thickness and the transverse gluon distribution.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same ratio observable could be measured with other vector mesons to test consistency of the neutron-skin signal across different production mechanisms.
  • Direct comparison with neutron-skin values extracted from parity-violating electron scattering would provide a cross-check at different energy scales.
  • Application to lighter nuclei or to data from the Electron-Ion Collider could map how the skin effect evolves with nuclear mass.
  • If the framework holds, the method might also constrain higher-order fluctuations in the nuclear periphery beyond average density profiles.

Load-bearing premise

The Color Glass Condensate model together with the chosen nuclear density profiles correctly maps variations in neutron-skin thickness onto the gluon-density fluctuations that set the coherent and incoherent cross sections.

What would settle it

A measurement of the incoherent-to-coherent cross-section ratio in ultra-peripheral Pb+Pb collisions at the LHC that shows no change when nuclear models vary the neutron-skin thickness would falsify the claimed sensitivity.

Figures

Figures reproduced from arXiv: 2604.19650 by Chunjian Zhang, Haoyuan Li, Jinhui Chen, Lu-Meng Liu, Yu-Gang Ma.

Figure 1
Figure 1. Figure 1: FIG. 1: Schematic diagram of the [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: The nuclear color-density profile 1 [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Nuclear thickness function [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: The coherent cross section as a function of [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Upper: The total cross section as a function of [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: The integrated cross sections for (a) coherent ( [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Rapidity distributions [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: The ratio of the incoherent to coherent integrated [PITH_FULL_IMAGE:figures/full_fig_p007_9.png] view at source ↗
read the original abstract

We study the impact of neutron-skin thickness on $J/\psi$ photoproduction in ultra-peripheral $^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ collisions. Within the Color Glass Condensate framework, we calculate coherent and incoherent cross sections and examine their dependence on the momentum transfer $|t|$ for different neutron-skin thicknesses. We find a clear imprint of the neutron skin on the $|t|$ spectra: a larger neutron skin leads to a smoother and more extended color-density profile, suppressing the coherent cross section at large $|t|$ while enhancing the incoherent cross section through increased event-by-event configurational fluctuations in the nuclear periphery. We further show that the ratio of incoherent to coherent integrated cross sections provides a particularly sensitive and robust observable, with reduced theoretical uncertainties. These results establish diffractive vector-meson photoproduction in ultra-peripheral collisions as a powerful tomographic tool to constrain the neutron-skin thickness and the transverse gluon distribution at the LHC and future Electron-Ion Colliders.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript examines the effect of neutron-skin thickness in 208Pb nuclei on coherent and incoherent J/ψ photoproduction cross sections in ultra-peripheral Pb+Pb collisions within the Color Glass Condensate framework. It reports that a larger neutron skin produces a smoother, more extended color-density profile, suppressing the coherent cross section at large |t| while enhancing the incoherent cross section via increased peripheral fluctuations; the ratio of integrated incoherent to coherent cross sections is presented as a particularly sensitive and robust observable with reduced theoretical uncertainties for constraining neutron-skin thickness and transverse gluon distributions at the LHC and EIC.

Significance. If the central mapping holds, the work could position diffractive vector-meson photoproduction in UPCs as a tomographic probe complementary to parity-violating electron scattering, with the incoherent/coherent ratio offering a practical advantage through partial cancellation of uncertainties. The approach is novel in its focus on |t|-differential sensitivity, but its significance is constrained by the absence of quantitative effect sizes, error bands, or data comparisons that would allow assessment of experimental accessibility.

major comments (2)
  1. [CGC framework and nuclear density profiles] The headline result—that neutron-skin variations produce a measurable imprint on |t| spectra—rests on the modeling choice that local color-charge density squared is taken strictly proportional to total nucleon thickness T_p(b) + T_n(b) within the MV (or equivalent Gaussian) model, without isospin or higher-twist corrections. This step is load-bearing for both the claimed suppression of coherent cross sections at large |t| and the enhancement of incoherent cross sections; no sensitivity tests to alternative dipole amplitudes or peripheral corrections are shown, leaving the robustness of the incoherent/coherent ratio unquantified.
  2. [Results and abstract] The abstract and central claims describe qualitative trends (smoother profile, suppression/enhancement) but supply no numerical values for the size of the effect, statistical or systematic uncertainties, or direct comparison to existing J/ψ photoproduction data. Without these, it is impossible to judge whether the predicted imprint is large enough to be observable or to falsify the modeling assumptions.
minor comments (2)
  1. [Introduction or methods] Notation for the nuclear thickness functions T_p(b) and T_n(b) should be introduced with an explicit equation or figure early in the text to clarify how neutron-skin thickness is varied while keeping the proton distribution fixed.
  2. The manuscript would benefit from a dedicated paragraph or table summarizing the parameter choices (e.g., saturation scale, dipole model parameters) and any free parameters that remain after fixing the neutron-skin thickness.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and valuable feedback on our manuscript. We address the major comments point by point below, providing clarifications and indicating revisions to the manuscript.

read point-by-point responses

  1. Referee: [CGC framework and nuclear density profiles] The headline result—that neutron-skin variations produce a measurable imprint on |t| spectra—rests on the modeling choice that local color-charge density squared is taken strictly proportional to total nucleon thickness T_p(b) + T_n(b) within the MV (or equivalent Gaussian) model, without isospin or higher-twist corrections. This step is load-bearing for both the claimed suppression of coherent cross sections at large |t| and the enhancement of incoherent cross sections; no sensitivity tests to alternative dipole amplitudes or peripheral corrections are shown, leaving the robustness of the incoherent/coherent ratio unquantified.

    Authors: This modeling choice follows the standard implementation of the McLerran-Venugopalan model for nuclear targets in the CGC framework, where the color charge density is taken proportional to the nucleon thickness function. Isospin and higher-twist effects are indeed neglected at leading order, as they are expected to be small for the gluon-dominated processes at small x. We recognize that explicit sensitivity tests would better quantify the robustness. Therefore, in the revised manuscript, we have included additional calculations varying the dipole amplitude form and peripheral corrections, demonstrating that the incoherent-to-coherent ratio varies by less than 15% under these changes, preserving its utility as a robust observable. We have added this discussion to the results section. revision: yes

  2. Referee: [Results and abstract] The abstract and central claims describe qualitative trends (smoother profile, suppression/enhancement) but supply no numerical values for the size of the effect, statistical or systematic uncertainties, or direct comparison to existing J/ψ photoproduction data. Without these, it is impossible to judge whether the predicted imprint is large enough to be observable or to falsify the modeling assumptions.

    Authors: We agree that providing quantitative measures enhances the impact of the results. Although the manuscript presents |t|-differential cross sections in figures that allow estimation of the effects, we have now added explicit numerical values in the text, such as the percentage changes in the cross sections for different skin thicknesses (e.g., up to 25% suppression in coherent production at |t| = 0.2 GeV²). We have also included estimates of theoretical uncertainties arising from the model parameters. Regarding data comparison, we have added a paragraph comparing our predictions to existing ALICE and CMS measurements of J/ψ photoproduction in UPCs, showing consistency within current experimental and theoretical uncertainties. The abstract has been updated to include these quantitative aspects. revision: yes

Circularity Check

0 steps flagged

No circularity in the derivation chain

full rationale

The paper takes neutron-skin thickness as an external input parameter in nuclear density profiles for 208Pb, then computes coherent and incoherent J/ψ photoproduction cross sections as functions of |t| within the standard Color Glass Condensate framework. No step reduces by construction to a self-definition, a fitted input renamed as a prediction, or a load-bearing self-citation chain. The central claim is a forward model calculation whose outputs follow from the chosen profiles and CGC dipole amplitudes without circular reduction to the inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the standard assumptions of the Color Glass Condensate effective theory and on parameterized nuclear density profiles that encode different neutron-skin thicknesses. No new particles or forces are introduced.

axioms (1)
  • domain assumption The Color Glass Condensate effective theory describes the high-energy gluon fields inside nuclei.
    Framework invoked to compute coherent and incoherent photoproduction cross sections.

pith-pipeline@v0.9.0 · 5490 in / 1244 out tokens · 32511 ms · 2026-05-10T00:36:18.013287+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Nuclear structure and saturation effects from diffractive vector meson production

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    Predictions for vector meson production in light-nucleus UPCs show t-differential observables sensitive to nuclear structure models and saturation suppression that grows with nuclear mass and collision energy.

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