arxiv: 2604.17685 · v1 · submitted 2026-04-20 · ⚛️ nucl-th

Recognition: unknown

Constraining neutron skin impurity in ⁴⁸Ca and its relevance for the CREX-PREX puzzle

Phan Nhut Huan

Authors on Pith no claims yet

Pith reviewed 2026-05-10 04:21 UTC · model grok-4.3

classification ⚛️ nucl-th

keywords neutron skin48CaCoulomb polarizationCREXPREXisobaric analog statecharge-exchange reactionsymmetry potential

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The pith

Coulomb core polarization mixes protons into the neutron skin of 48Ca, enhancing the apparent neutron distribution by 0.052 fm in thin-skin models.

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

The paper investigates how electric repulsion among protons in calcium-48 pushes some protons outward, creating an admixture of protons inside the neutron skin region. This mixing occurs because the radius where Coulomb effects start driving protons out lies inside the core, before the skin begins. For a skin thickness of 0.144 fm that aligns with the CREX thin-skin measurement, the admixture triggers a symmetry potential that effectively increases the neutron distribution by 0.052 fm. The impurity effect weakens as skin thickness grows, because stronger symmetry forces counteract the proton shift. The work proposes isobaric analog state charge-exchange reactions as a way to measure this thickness-dependent impurity directly.

Core claim

Coulomb-driven proton shifts in 48Ca produce neutron skin impurity, defined as protons present in the neutron skin region. The Coulomb boundary radius sits before the skin, making the admixture intrinsic to the density profile. For a 0.144 fm neutron skin consistent with CREX, this impurity enhances the probed neutron distribution by approximately 0.052 fm through the symmetry potential response. The enhancement is suppressed in thick-skin cases where the symmetry potential neutralizes core polarization.

What carries the argument

Neutron skin impurity, the proton admixture in the neutron skin region induced by Coulomb core polarization, which then elicits a symmetry-potential response that modifies the neutron density.

If this is right

Neutron skin impurity is suppressed in large-skin scenarios because the symmetry potential neutralizes core polarization.
IAS charge-exchange measurements at 420 MeV can quantify impurity-induced changes to the neutron distribution.
The thickness dependence of the impurity offers a way to distinguish thin-skin from thick-skin models without relying solely on parity-violating electron scattering.

Where Pith is reading between the lines

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

The same Coulomb-induced mixing could appear in other calcium isotopes or nearby nuclei where neutron skins are measured.
Varying the beam energy in the (3He,t) reaction might separate the impurity contribution from the underlying skin thickness.
If the impurity effect holds, it supplies a conventional nuclear-structure explanation for part of the CREX-PREX tension.

Load-bearing premise

The Coulomb boundary radius lies before the neutron skin region, so proton admixture is always present and its main effect is a thickness-dependent symmetry-potential enhancement of the neutron distribution.

What would settle it

An independent measurement of the proton density profile in the outer region of 48Ca that shows zero protons in the nominal neutron skin zone would falsify the impurity mechanism.

Figures

Figures reproduced from arXiv: 2604.17685 by Phan Nhut Huan.

**Figure 2.** Figure 2: FIG. 2. Differential cross sections of the [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3. (a) Coulomb core-polarization distribution of [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 4.** Figure 4: FIG. 4. (a) Coulomb core-polarization distribution of [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗

read the original abstract

The impact of Coulomb core polarization on the neutron density distribution of $^{48}\mathrm{Ca}$ is investigated, revealing that Coulomb-driven proton shifts toward the nuclear surface lead to neutron skin impurity, defined as the presence of protons in the neutron skin region. The Coulomb boundary radius, where protons begin to be driven outward, is located before the neutron skin region, leading to proton admixture and establishing impurity as an intrinsic feature of the density profile in $^{48}\mathrm{Ca}$. Using the $(^3\mathrm{He},t)$ isobaric analog state (IAS) reaction at 420 MeV, we quantify impurity-induced modifications of the probed neutron distribution. For a neutron skin thickness of 0.144 fm, consistent with the thin-skin scenario suggested by CREX ($0.121 \pm 0.035$ fm), Coulomb core polarization effectively enhances the neutron distribution by approximately 0.052 fm via the symmetry potential response to proton admixture. Our analysis shows that neutron skin impurity is thickness-dependent and becomes suppressed in large-skin scenarios as the symmetry potential neutralizes core polarization, thereby motivating IAS charge-exchange measurements as a complementary probe of neutron skin observables.

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

The paper frames Coulomb polarization as creating an intrinsic 'neutron skin impurity' that boosts the effective neutron distribution by 0.052 fm in thin-skin 48Ca, but the number and its thickness dependence rest on an unshown model and a key radial assumption.

read the letter

The paper's central claim is a 0.052 fm enhancement to the neutron distribution in 48Ca from Coulomb-driven proton admixture when the skin is thin, matching CREX. This is presented as an intrinsic impurity effect that weakens for thicker skins. What stands out as new is the explicit definition of neutron skin impurity and its thickness dependence, plus the tie-in to IAS charge-exchange as a probe. The abstract does a decent job highlighting how this could address the CREX-PREX discrepancy through the symmetry potential response. The work connects established ideas about core polarization to the current neutron skin measurements in a direct way. Suggesting a specific reaction energy and type for testing is practical. On the downside, the quantitative result lacks any supporting derivation or model specification. The 0.052 fm value is given for an input skin thickness chosen to align with CREX data, which makes the thickness dependence look potentially tautological rather than independently tested. The assumption that the Coulomb boundary precedes the neutron skin region is stated but not backed by radial density comparisons or sensitivity checks, which is the load-bearing part according to the stress test. If that boundary condition doesn't hold in the actual density profiles used, the admixture and the enhancement both go away. The paper would be stronger with explicit figures showing the proton and neutron densities and where the boundary falls. This is for people tracking the symmetry energy constraints from parity-violating electron scattering and related probes. A reader interested in the CREX-PREX puzzle will get a new angle to consider, though they'll likely want the full calculation details. It has enough of a novel framing and experimental suggestion to warrant peer review rather than a desk reject. I'd send it on, but flag the need for the reaction model and density plots in the revisions.

Referee Report

3 major / 2 minor

Summary. The manuscript investigates Coulomb core polarization effects on the neutron density in 48Ca, introducing the concept of 'neutron skin impurity' as proton admixture in the neutron skin region. It posits that the Coulomb boundary radius precedes the skin onset, rendering impurity intrinsic and thickness-dependent. For a neutron skin thickness of 0.144 fm (aligned with CREX thin-skin results), the paper claims this leads to an ~0.052 fm enhancement of the probed neutron distribution via symmetry-potential response in the (3He,t) IAS reaction at 420 MeV; the effect is said to be suppressed for thicker skins, motivating IAS measurements as a complementary probe to resolve the CREX-PREX puzzle.

Significance. If the central quantitative claims hold after detailed validation, the work could offer a mechanism to reconcile thin-skin CREX data with other observables by showing thickness-dependent Coulomb modifications to neutron distributions. The introduction of neutron skin impurity as an intrinsic feature adds a conceptual angle on isovector responses in light nuclei, and the suggestion of IAS charge-exchange as a probe has potential experimental relevance. Strengths include the focus on a falsifiable prediction tied to existing CREX constraints.

major comments (3)

[Abstract] Abstract and main derivation: the stated 0.052 fm enhancement for 0.144 fm skin thickness is presented as a precise numerical result, yet no explicit reaction model, density functional, radial density profiles, or error propagation is referenced to derive or support this value; without these, the quantitative claim cannot be verified as independent of input assumptions.
[Abstract] The central assumption that the Coulomb boundary radius lies before the neutron skin region (enabling thickness-dependent proton admixture and symmetry-potential enhancement) is load-bearing for both the 0.052 fm shift and the suppression in thick-skin scenarios, but no explicit comparison of charge vs. neutron density radii or boundary location is provided; if this positioning does not hold in the model's densities, the admixture vanishes and the thickness dependence reverses.
[Abstract] The quantification of impurity-induced modifications via the (3He,t) IAS reaction lacks details on the optical potential, distortion effects, or how the enhancement is extracted from cross sections, making it unclear whether the 0.052 fm figure is a model output or a restatement of the input skin thickness chosen to match CREX.

minor comments (2)

[Abstract] The abstract introduces 'neutron skin impurity' without a concise definition, which could be clarified for readers.
Consider adding a figure or table explicitly showing the radial positions of the Coulomb boundary, proton and neutron densities, and skin region to substantiate the key geometric assumption.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the thoughtful and detailed comments on our manuscript. The points raised concern the clarity and support for the quantitative claims in the abstract and main text. We address each comment below and have revised the manuscript to incorporate additional details and clarifications where necessary.

read point-by-point responses

Referee: [Abstract] Abstract and main derivation: the stated 0.052 fm enhancement for 0.144 fm skin thickness is presented as a precise numerical result, yet no explicit reaction model, density functional, radial density profiles, or error propagation is referenced to derive or support this value; without these, the quantitative claim cannot be verified as independent of input assumptions.

Authors: The 0.052 fm value is obtained from our model calculations of the symmetry potential response to the proton admixture induced by Coulomb core polarization, as detailed in the main text using the specified density functional and reaction framework for the (3He,t) IAS reaction. We acknowledge that the abstract does not reference these elements explicitly. In the revised manuscript, we will update the abstract to briefly indicate the underlying approach and direct readers to the relevant sections and figures for the derivation, including radial profiles and the extraction method. revision: yes
Referee: [Abstract] The central assumption that the Coulomb boundary radius lies before the neutron skin region (enabling thickness-dependent proton admixture and symmetry-potential enhancement) is load-bearing for both the 0.052 fm shift and the suppression in thick-skin scenarios, but no explicit comparison of charge vs. neutron density radii or boundary location is provided; if this positioning does not hold in the model's densities, the admixture vanishes and the thickness dependence reverses.

Authors: The manuscript includes radial density profiles demonstrating that the Coulomb boundary radius precedes the neutron skin region for the thin-skin case. This positioning is central to our analysis and is shown to lead to the thickness-dependent impurity. To address the concern, we will add an explicit textual comparison of the charge and neutron density radii in the revised version, including a statement confirming the boundary location relative to the skin onset. revision: yes
Referee: [Abstract] The quantification of impurity-induced modifications via the (3He,t) IAS reaction lacks details on the optical potential, distortion effects, or how the enhancement is extracted from cross sections, making it unclear whether the 0.052 fm figure is a model output or a restatement of the input skin thickness chosen to match CREX.

Authors: The enhancement is a calculated output from the reaction model, where the effective neutron distribution is modified by the symmetry potential acting on the proton impurity, leading to the 0.052 fm shift for the input skin thickness of 0.144 fm. Details on the optical potentials and distortion effects are provided in the methods section of the manuscript. The 0.052 fm is not a restatement but results from the response calculation. We will revise the abstract to clarify this distinction and reference the extraction procedure from the cross sections. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected; derivation is model-based and self-contained

full rationale

The paper adopts the neutron skin thickness value of 0.144 fm directly from external CREX measurements as an input parameter and then computes the resulting Coulomb-driven enhancement (0.052 fm) via a symmetry-potential model applied to the density profiles. The statement that the Coulomb boundary radius precedes the neutron skin region is presented as a direct consequence of inspecting those profiles rather than a definitional premise that forces the numerical outcome. No parameters are fitted to a data subset and then relabeled as predictions, no self-citations supply load-bearing uniqueness theorems, and the thickness dependence is explored by varying the input skin thickness within the same model framework. The central quantitative claim therefore remains an independent model output rather than a restatement of the chosen input by construction.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 1 invented entities

The central claim rests on standard nuclear-physics assumptions about Coulomb polarization and symmetry potentials together with the newly introduced descriptive entity of neutron skin impurity; no independent experimental handle for the impurity is supplied.

free parameters (1)

neutron skin thickness = 0.144 fm
Input value of 0.144 fm selected to lie inside the CREX uncertainty band for the numerical example.

axioms (2)

domain assumption Coulomb core polarization drives protons outward from the nuclear interior
Invoked to locate the Coulomb boundary radius before the neutron skin and thereby produce intrinsic impurity.
domain assumption Symmetry potential responds to proton admixture by redistributing neutrons outward
Used to convert the impurity into the stated 0.052 fm enhancement of the probed neutron distribution.

invented entities (1)

neutron skin impurity no independent evidence
purpose: Descriptive label for the proton admixture inside the nominal neutron skin region caused by Coulomb polarization
Introduced to characterize the admixture as an intrinsic, thickness-dependent feature of the density profile.

pith-pipeline@v0.9.0 · 5503 in / 1719 out tokens · 94023 ms · 2026-05-10T04:21:23.880398+00:00 · methodology

discussion (0)

Reference graph

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Panels (a) and (b) present the core polarization distribution and the corresponding t ran- sition potentials for SAMi-J29. In this scenario, the Coulo mb boundary radius shifts inward to 3.18 fm due to the increasin g dominance of the symmetry potential, while the convergence point between the two transition potentials is located at 3. 51 fm, resulting in...
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