Neutron skins: A perspective from dispersive optical models

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.