CREX and PREX-II reconciled within energy-density functional theory

Abstract

The CREX and PREX-II measurements of the neutron-skin thickness of 48Ca and 208Pb challenge standard nuclear energy-density functional (EDF) descriptions of nuclei and nuclear matter. We show that the apparent tension arises from an implicit constraint in EDF theory, which ties the density dependence of the functional at the dilute nuclear surface to that of uniform matter near saturation. Relaxing this surface-bulk coupling and independently constraining the dilute-density sector, while preserving realistic saturation and high-density behavior, yields EDFs that simultaneously reproduce the neutron skins of both nuclei, their electric dipole polarizabilities, and neutron-star mass-radius relations. The established correlation between the neutron-skin thickness and the symmetry-energy slope parameter L at saturation is retained but becomes substantially broader. The results show that current neutron-skin data do not require extreme values of L and highlight an underconstrained degree of freedom associated with low-density nuclear matter.

Figures (3)

• Figure 1: Neutron-skin thickness of $^{48}$Ca and $^{208}$Pb calculated with EDFs based on the shown bulk EoSs and satisfying ADPD$(19)\leq 1.5\%$. The rectangle encloses the reported experimental values. The curves correspond to posterior distributions (arbitrary common scale). Dotted lines:$V_d=0$ and varying $m^{\ast}/m,\kappa,W_0,D_{12}$; the results for $^{208}$Pb are divided by 5 for visibility. Points and full lines: Fixed $m^{\ast}/m,\kappa,W_0,D_{12}$ and varying $V_d$; points within the rectangle are highlighted.
• Figure 2: Dipole strength distributions obtained with APR(B), APR(C), and QMC(A), satisfying all four constraints, and APR(A), satisfying CREX, PREX-II, and $a_D(^{208}\mathrm{Pb})$. Data are from the M0636 ($^{48}$Ca) and L0021 ($^{208}$Pb) data files of the EXFOR database.
• Figure 3: Energy per particle of symmetric nuclear matter (SNM) and symmetry energy for APR- and QMC-based EDFs (red and blue, respectively) before (dotted lines) and after (full lines) introducing the low-density correction. QMC(A) is highlighted with full blue lines. The inset shows the respective neutron star mass-radius relations.