Evidence for strong isovector nuclear spin-orbit interaction

Abstract

The nucleon spin-orbit interaction is a cornerstone of nuclear structure theory, yet its isospin dependence remains elusive owing to the lack of clean experimental probes. Here we show that the charge-weak form factor difference in $^{48}$Ca, recently extracted in a model-independent manner by the CREX experiment, exhibits strong sensitivity to the isovector spin-orbit interaction. Using Skyrme-like energy density functionals, we demonstrate that a significantly enhanced isovector spin-orbit interaction, about four times stronger than conventional parametrizations, can resolve the PREX-CREX puzzle, which has challenged modern nuclear theories and our understanding of nuclear symmetry energy, while maintaining a good description of nuclear bulk properties and well-established shell structure of finite nuclei. This enhanced isovector spin-orbit interaction also provides a novel mechanism for the emergence of the $N = 14$, $16$, $32$ and $34$ magic numbers in neutron-rich nuclei on the mean-field level. These findings point to a strong isospin dependence of the nucleon spin-orbit interaction, which is expected to have important implications for nuclear structures, electroweak nuclear processes, and related problems in nuclear astrophysics.