arxiv: 2604.08136 · v1 · submitted 2026-04-09 · ⚛️ nucl-th

Recognition: unknown

The neutron skin effect in Pb+Pb collisions at 2.76A TeV at the LHC

Amit Paul , Rupa Chatterjee

Authors on Pith no claims yet

Pith reviewed 2026-05-10 17:11 UTC · model grok-4.3

classification ⚛️ nucl-th

keywords neutron skin thicknessPb+Pb collisionselliptic flowinitial spatial anisotropyLHCphotonsheavy-ion collisionsquark-gluon plasma

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The pith

Neutron skin in lead nuclei increases the initial spatial anisotropy in peripheral Pb+Pb collisions at the LHC, enhancing elliptic flow especially for photons.

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

The paper examines how the neutron skin thickness in lead nuclei, arising from the extended neutron density compared to protons, shapes the early geometry of the fireball in relativistic heavy-ion collisions. By varying this thickness in the initial nuclear density profiles and evolving the system, the authors track changes in average transverse momentum, spectra, and flow coefficients at 2.76A TeV. A sympathetic reader would care because the altered overlap asymmetry in glancing collisions produces clear signals in final observables that experiments can measure. This links nuclear structure details directly to the dynamics of the hot dense medium created at the LHC.

Core claim

The initial spatial anisotropy of the fireball is affected by the neutron skin thickness significantly especially for the peripheral collisions. This leads to a substantial enhancement of the elliptic flow of hadrons with an even stronger effect observed for photons. In addition, the effect is found to be more pronounced for lower beam energy collisions of lead nuclei.

What carries the argument

Neutron skin thickness incorporated into the initial nuclear density distributions of lead nuclei, which modifies the spatial anisotropy of the fireball overlap region and propagates through the space-time evolution to final observables.

Load-bearing premise

The neutron skin thickness is correctly incorporated into the initial nuclear density distributions used for the collision simulations, and the subsequent evolution model accurately propagates this effect to final observables without being dominated by other model uncertainties.

What would settle it

If elliptic flow measurements for hadrons and photons in peripheral Pb+Pb collisions at 2.76 TeV show no difference between simulations with and without neutron skin, the claimed significant enhancement from the skin would be ruled out.

Figures

Figures reproduced from arXiv: 2604.08136 by Amit Paul, Rupa Chatterjee.

**Figure 2.** Figure 2: FIG. 2: (Color online) Centrality dependent initial spatia [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3: (Color online) Time evolution of spatial eccentrici [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

**Figure 4.** Figure 4: FIG. 4: Constant energy density contours (in GeV) along [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗

**Figure 6.** Figure 6: FIG. 6: (Color online) Charged pion spectra calculated with [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗

**Figure 7.** Figure 7: FIG. 7: (Color online) Average [PITH_FULL_IMAGE:figures/full_fig_p005_7.png] view at source ↗

**Figure 9.** Figure 9: FIG. 9: (Color online) Thermal photon [PITH_FULL_IMAGE:figures/full_fig_p006_9.png] view at source ↗

**Figure 10.** Figure 10: FIG. 10: (Color online) Elliptic flow of thermal photons [PITH_FULL_IMAGE:figures/full_fig_p006_10.png] view at source ↗

read the original abstract

Collisions of lead nuclei at relativistic energies provide valuable insight into the properties of the quark gluon plasma formed in such collisions where the initial geometry and density profile play a crucial role in governing the subsequent evolution of the produced hot and dense fireball. The neutron skin thickness resulting from the difference between the neutron and proton density distributions in neutron rich lead nuclei plays an important role in nuclear structure studies. In this work we investigate the impact of neutron skin on the space time evolution of the fireball formed in Pb+Pb collisions at 2.76A TeV at the LHC and analyze how the presence of neutron skin affect bulk observables sensitive to the initial nuclear structure. The time evolution of initial profile along with the average $p_T$, particle spectra and anisotropic flow parameters are estimated to investigate the effect of neutron skin on these observables. The initial spatial anisotropy of the fireball is found to be affected by the neutron skin thickness significantly especially for the peripheral collisions. This leads to a substantial enhancement of the elliptic flow of hadrons with an even stronger effect observed for photons. In addition, the effect is found to be more pronounced for lower beam energy collisions of lead nuclei.

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.

Desk Editor's Note private letter to a colleague

Neutron skin raises initial anisotropy and v2 in peripheral Pb+Pb, but the size of the effect looks sensitive to untested hydro choices.

read the letter

The paper runs standard hydrodynamic simulations of 2.76 TeV Pb+Pb collisions with and without a neutron skin in the lead nuclei. It reports that the skin increases the initial spatial eccentricity, most clearly in peripheral events, and that this feeds through to a noticeable rise in elliptic flow for hadrons and a larger rise for photons. The effect is also said to grow at lower beam energies. That is the concrete new piece: a direct comparison of bulk observables with and without the skin at LHC energy, including direct photons as an extra probe. The calculation itself is straightforward and follows the usual pipeline from modified nuclear density profiles through evolution to spectra and flow coefficients. The authors track how the initial profile changes with time and show the resulting shifts in average pT and anisotropy parameters. This kind of targeted check on initial-condition ingredients is useful for people who extract transport coefficients from heavy-ion data. The soft spot is exactly the one the stress-test note flags. The abstract and the reported findings give no sign that shear viscosity, particlization temperature, or initial-state fluctuations were varied while the skin thickness was held fixed. Peripheral collisions are small and short-lived, so v2 is known to be sensitive to those choices. Without those tests it is hard to know whether the reported enhancement is a robust consequence of the skin or an artifact of the default model settings. The neutron-skin implementation itself is described only at the level of “difference between neutron and proton density distributions,” with no quantitative table of the thickness values used or comparison to modern nuclear-structure calculations. The work is therefore a useful incremental study rather than a definitive one. People who build initial conditions for heavy-ion codes or who care about nuclear-structure input to QGP phenomenology would get something out of it. It is not a load-bearing claim that would change the field, but it is a clean enough calculation to deserve referee time so the robustness questions can be settled in the published version.

Referee Report

2 major / 1 minor

Summary. The manuscript investigates the role of neutron skin thickness in lead nuclei on the initial conditions and space-time evolution of the fireball in Pb+Pb collisions at 2.76A TeV. It reports that the neutron skin modifies the initial spatial anisotropy significantly, particularly in peripheral collisions, resulting in a substantial enhancement of elliptic flow for hadrons and an even stronger effect for photons, with the impact being more pronounced at lower beam energies.

Significance. If the central claim holds after robustness checks, the work would usefully connect nuclear structure (neutron skin) to heavy-ion observables, especially via direct photons which probe early dynamics. The reported peripheral sensitivity could motivate new experimental searches, but the absence of quantitative benchmarks, error estimates, and model-variation studies in the current presentation limits its immediate impact.

major comments (2)

[Abstract] Abstract: the central claim of a 'substantial enhancement' of elliptic flow (stronger for photons) due to neutron skin is stated without any numerical values, ratios, figures, or baseline comparisons, preventing assessment of whether the effect survives hydrodynamic evolution or is comparable to known uncertainties in peripheral collisions.
[Results and discussion] The manuscript provides no evidence that hydrodynamic parameters (shear viscosity, equation of state, particlization temperature, or initial-state fluctuations) were systematically varied while holding neutron-skin thickness fixed. Given that v2 in peripheral systems is highly sensitive to these choices, the reported enhancement cannot yet be attributed robustly to the neutron skin rather than default model settings.

minor comments (1)

[Abstract] The abstract mentions 'average p_T, particle spectra and anisotropic flow parameters' but does not specify the hydrodynamic code, initial-condition generator, or neutron-skin parametrization employed; these details are required for reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We appreciate the referee's insightful comments on our manuscript. The suggestions regarding the abstract and the need for robustness checks against hydrodynamic parameter variations are well taken. We will revise the manuscript to incorporate quantitative details in the abstract and provide additional discussion on the sensitivity to model parameters to strengthen our conclusions.

read point-by-point responses

Referee: [Abstract] Abstract: the central claim of a 'substantial enhancement' of elliptic flow (stronger for photons) due to neutron skin is stated without any numerical values, ratios, figures, or baseline comparisons, preventing assessment of whether the effect survives hydrodynamic evolution or is comparable to known uncertainties in peripheral collisions.

Authors: We agree with the referee that the abstract should provide more concrete information to allow readers to assess the magnitude of the effect. In the revised manuscript, we will update the abstract to include specific numerical values and ratios for the enhancement in elliptic flow of hadrons and photons, particularly in peripheral collisions, and reference the corresponding figures and baseline comparisons from our results. revision: yes
Referee: [Results and discussion] The manuscript provides no evidence that hydrodynamic parameters (shear viscosity, equation of state, particlization temperature, or initial-state fluctuations) were systematically varied while holding neutron-skin thickness fixed. Given that v2 in peripheral systems is highly sensitive to these choices, the reported enhancement cannot yet be attributed robustly to the neutron skin rather than default model settings.

Authors: The referee correctly points out that we have not systematically varied the hydrodynamic parameters in the current study. Our calculations were performed with a fixed set of standard parameters to isolate the impact of the neutron skin on the initial conditions. We recognize that this limits the robustness claim. In the revised version, we will include a new subsection discussing the potential effects of varying shear viscosity, equation of state, and other parameters, and present arguments based on the initial geometry change that the enhancement should be largely independent of these choices. However, a full parameter scan is beyond the scope of the current work. revision: partial

Circularity Check

0 steps flagged

No circularity: standard forward simulation from input nuclear densities to computed observables

full rationale

The paper modifies the initial nuclear density profiles of Pb nuclei to include a neutron skin thickness (an external nuclear-structure input), then evolves the system via a hydrodynamic or transport model to obtain space-time profiles, spectra, and anisotropic flow coefficients. All reported effects on elliptic flow and photon v2 are direct numerical outputs of this forward chain; no target observable is used to fit a parameter that is later renamed as a prediction, and no load-bearing step reduces by definition or self-citation to the final result. The derivation remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions in heavy-ion physics modeling with the addition of neutron skin in initial conditions. Specific values and other parameters not detailed in the abstract.

free parameters (1)

neutron skin thickness parameter
The thickness is a key input from nuclear models that is varied or fixed to study the effect.

axioms (1)

domain assumption Relativistic hydrodynamic evolution of the QGP fireball
The space-time evolution is modeled using standard hydrodynamic equations for the hot dense matter.

pith-pipeline@v0.9.0 · 5503 in / 1286 out tokens · 80150 ms · 2026-05-10T17:11:59.765871+00:00 · methodology

discussion (0)

Reference graph

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