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The recent PREX-2 and CREX data on the model-independent extraction of the charge-weak form factor difference F ₂ₖ in ^208Pb and ^48Ca challenge modern nuclear energy density functionals (EDFs) as well as our present understanding on the neutron skin and nuclear symmetry energy. Within the Skyrme-like EDFs, we demonstrate that the isovector spin-orbit interaction can strongly change the F ₂ₖ in ^48Ca while it has essentially no influence on the F ₂ₖ in ^208Pb, mainly due to the eight spin-orbit unpaired 1f₇/₂ neutrons in ^48Ca. To simultaneously describe PREX-2 and CREX data in 1 error, we find the strength of isovector spin-orbit interaction should be larger than about four times of that in the conventional Skyrme-like EDFs, implying the neutrons and protons have significantly different spin-orbit interaction. To further reconcile the data on electric dipole polarizability in ^208Pb and ^48Ca, we obtain L 55 MeV for the slope parameter of the symmetry energy, r ₍ (^208Pb) 0. 19 fm and r ₍ (^48Ca) 0. 12 fm for the neutron skin thickness. The implications of the strong isovector spin-orbit interaction are discussed.
Yue et al. (Thu,) studied this question.