Neutron skins: A perspective from dispersive optical models

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 ⁴⁸Ca (R⁴⁸ₛkin=0.25 ± 0.023 fm in 2017) and ²⁰⁸Pb (R²⁰⁸ₛkin=0.25 ± 0.05 fm in 2020). While the DOM prediction in ²⁰⁸Pb is within 1σ of the subsequent PREX-2 measurement, the DOM prediction in ⁴⁸Ca is over 2σ larger than the thin neutron skin resulting from CREX. From the moment it was revealed, the thin skin in ⁴⁸Ca has puzzled the nuclear-physics community as no adequate theories simultaneously predict both a large skin in ²⁰⁸Pb and a small skin in ⁴⁸Ca. The DOM is unique in its ability to treat both structure and reaction data on the same footing, providing a unique perspective on this Rₛkin puzzle. It appears vital that more neutron data be measured in both the scattering and bound-state domain for ⁴⁸Ca to clarify the situation.