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.

Figures (2)

• Figure 1: Charge-weak form-factor differences in $^{208}$Pb and $^{48}$Ca. Predictions from the three Skyrme-like EDFs eS53, eS250, and eS500$_{\rm T}$ constructed in this work are shown as open stars. Also included are predictions from various covariant (squares) and nonrelativistic (diamonds) EDFs CREX:2022kggYue:2021yfx. The ellipses depict the joint PREX-II and CREX $67\%$ and $90\%$ probability contours, while the rectangle indicates the marginal $68.3\%$ confidence intervals for $\Delta F_{\rm CW}^{48}$ and $\Delta F_{\rm CW}^{208}$CREX:2022kgg.
• Figure 2: Two-neutron shell gaps $\Delta_{2\rm{n}}$ for O (a) and Ca (b) isotopes. Predictions of eS250 and eS53 are compared with experimental data Wang:2021xhn and with the Skyrme EDFs SLy4 Chabanat:1997un, UNEDF0 Kortelainen:2010hv, and UNEDF1 Kortelainen:2011ft.