Neutron skins of 208Pb and 48Ca from parity-violating scattering
In plain words
In heavy nuclei the extra neutrons form a thin outer skin whose thickness tells how the pressure of neutron-rich matter rises with density. Two Jefferson Lab experiments give values for lead and calcium that most nuclear models cannot fit at the same time.
Precise statement
PREX-II gives $R_{\mathrm{skin}}(208\mathrm{Pb})$ = $R_{n} - R_{p}$ = 0.283 +/- 0.071 fm and CREX gives $R_{\mathrm{skin}}(48\mathrm{Ca})$ = 0.121 +/- 0.026 (exp) +/- 0.024 (model) fm. Determine whether a single nuclear Hamiltonian or energy density functional reproduces both measured parity-violating asymmetries $A_{\mathrm{PV}}$, and fix $R_{\mathrm{skin}}(208\mathrm{Pb})$ to +/- 0.03 fm. Answer: a consistent description, or the identified experimental or theoretical cause of the tension.
What would settle it
The MREX parity-violating experiment at the Mainz MESA accelerator, planned to reach roughly 0.03 fm on $R_{\mathrm{skin}}(208\mathrm{Pb})$, compared with ab initio predictions of both asymmetries.
Status in the literature
Unverified note
Ab initio calculations (Hu et al., Nature Physics 2022) predicted $R_{\mathrm{skin}}(208\mathrm{Pb})$ between 0.14 and 0.20 fm, compatible with PREX-II within about 1.5 standard deviations; the joint PREX-II and CREX tension for density functionals persisted as of 2026.