Evidence for strong isovector nuclear spin-orbit interaction

TL;DR

This paper provides evidence that the isovector component of the nuclear spin-orbit interaction is significantly stronger than previously thought, impacting nuclear structure models and our understanding of neutron-rich nuclei.

Contribution

It demonstrates that an enhanced isovector spin-orbit interaction explains experimental charge-weak form factor differences and resolves the PREX-CREX puzzle.

Findings

Enhanced isovector spin-orbit interaction resolves the PREX-CREX puzzle.

Strong isospin dependence of the nucleon spin-orbit interaction is supported.

Implications for nuclear structure, electroweak processes, and nuclear astrophysics.

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.