Constraining neutron skin impurity in $^{48}\mathrm{Ca}$ and its relevance for the CREX-PREX puzzle

TL;DR

This paper investigates how Coulomb core polarization causes proton admixture in the neutron skin of $^{48}$Ca, affecting neutron skin measurements and providing insights relevant to the CREX-PREX puzzle.

Contribution

It introduces the concept of neutron skin impurity caused by Coulomb-driven proton shifts and quantifies its impact on neutron distribution measurements in $^{48}$Ca.

Findings

Coulomb core polarization enhances neutron distribution by about 0.052 fm for a 0.144 fm neutron skin.

Proton admixture in the neutron skin is thickness-dependent and diminishes in large-skin scenarios.

IAS charge-exchange measurements can serve as a complementary probe for neutron skin observables.

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.