{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"167fb64e78929c3e5f574519baf3006f758ba606141f6570e0d0fe6e9d236f81","created":"2026-10-03T07:17:53Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"93e18bfadf36863b4ccddf643eed0c2921adc90c3fa973ceeac362632861d2c7","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"amo.bound-state-qed.helium-isotope-shift","kind":"well-posed","literature_status":"contested","n":"1","parents":[],"plain":"The difference in size between helium-3 and helium-4 nuclei can be measured with ordinary helium atoms or with muonic helium ions. The two methods disagree by about four standard errors.","posed_since":"2023","precise":"The squared charge-radius difference $r_{h}^{2} - r_{\\alpha}^{2}$ extracted from the $2^{1}\\mathrm{S}-2^{3}\\mathrm{S}$ isotope shift in electronic helium differs by $3.6\\, \\sigma$ from the muonic He-3 and He-4 Lamb-shift values (2023). Determine whether the discrepancy is due to missing atomic theory (hyperfine mixing, QED recoil), nuclear-structure (polarizability) corrections, or experiment.","problem_ref":null,"references":"","settled_by":"An independent measurement of the electronic helium isotope shift on a different transition, together with complete theory, giving $r_{h}^{2} - r_{\\alpha}^{2}$ in agreement with the muonic value.","status_note":"A 2024 chiral-EFT analysis of nuclear-structure corrections (arXiv 2401.13424) raised the tension to about $4\\,\\sigma$, while a late-2024 complete second-order hyperfine calculation (arXiv 2411.05621) reports that it largely resolves the disagreement; the two claims have not been reconciled.","title":"Helion-$\\alpha$ charge radius difference from electronic versus muonic 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