Neutron skins: A perspective from dispersive optical models

TL;DR

This paper reviews neutron skin predictions using a dispersive optical model that integrates scattering and bound-state data, highlighting discrepancies between predictions and recent measurements for calcium-48 and lead-208.

Contribution

It demonstrates the dispersive optical model's ability to unify structure and reaction data for neutron skin predictions, addressing longstanding puzzles in nuclear physics.

Findings

DOM predicts large neutron skins for ${}^{48}$Ca and ${}^{208}$Pb.

DOM prediction for ${}^{208}$Pb aligns with PREX-2 measurement.

DOM prediction for ${}^{48}$Ca exceeds CREX measurement.

Abstract

An overview is presented of neutron skin predictions obtained using an empirical nonlocal dispersive optical model (DOM). The DOM links both scattering and bound-state experimental data through a subtracted dispersion relation which allows for fully-consistent, data-informed predictions for nuclei where such data exists. Large skins were predicted for both ${}^{48}$Ca ($R^{48}_\textrm{skin}=0.25 \pm 0.023$ fm in 2017) and $^{208}$Pb ($R^{208}_\textrm{skin}=0.25 \pm 0.05$ fm in 2020). While the DOM prediction in $^{208}$Pb is within 1$\sigma$ of the subsequent PREX-2 measurement, the DOM prediction in $^{48}$Ca is over 2$\sigma$ larger than the thin neutron skin resulting from CREX. From the moment it was revealed, the thin skin in ${}^{48}$Ca has puzzled the nuclear-physics community as no adequate theories simultaneously predict both a large skin in ${}^{208}$Pb and a small skin in ${}^{48}$Ca. The DOM is unique in its ability to treat both structure and reaction data on the same footing, providing a unique perspective on this $R_\textrm{skin}$ puzzle. It appears vital that more neutron data be measured in both the scattering and bound-state domain for ${}^{48}$Ca to clarify the situation.