{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"0c70dcae2b0696b6d6f004429d1562d9aa394cbe534c505151960d6907d37b31","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":["ai:gpt-5.5"],"kind":"seed"},"parents":["6eeda053a8b70d3b04d4b6c2b0ac7ad8c196610fe332fee9e6dc87eb9bc58385","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"33fdcc3b66e75a7e52a9ac29a28c3c14017a5a705a8c5374b1d9f0ae2a7a66d8","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":["ai:gpt-5.5"],"external_id":"nuc.ab-initio-nuclei.lead-neutron-skin","kind":"well-posed","literature_status":"contested","n":"2","parents":[],"plain":"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.","posed_since":"2022","precise":"PREX-II gives $R_{\\mathrm{skin}}(208\\mathrm{Pb})$ = $R_{n} - R_{p}$ = $0.283 \\pm 0.071\\ \\mathrm{fm}$ and CREX gives $R_{\\mathrm{skin}}(48\\mathrm{Ca})$ = $0.121 \\pm 0.026\\ (\\mathrm{exp}) \\pm 0.024\\ (\\mathrm{model})\\ \\mathrm{fm}$. 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 $\\pm 0.03\\ \\mathrm{fm}$. Answer: a consistent description, or the identified experimental or theoretical cause of the tension.","problem_ref":"6eeda053a8b70d3b04d4b6c2b0ac7ad8c196610fe332fee9e6dc87eb9bc58385","references":"","settled_by":"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_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.","title":"Neutron skins of 208Pb and 48Ca from parity-violating 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