{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"f262f49de00ebff4bdeb36d4acd1be74e46c08364fc137211a407ea4e42e97df","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":["ai:gpt-5.5"],"kind":"seed"},"parents":["c8fb1852ecb4a0cd3bd5349236cf4d3c9cecb666980f4c0ebefc81995860a8f7","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"69ba5f58d08fb7b93f75ece0ae1c516fc0ae0f17e5733ed78790a70420373a16","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":["ai:gpt-5.5"],"external_id":"amo.bound-state-qed.proton-radius-residual","kind":"phenomenon","literature_status":"partially-resolved","n":"2","parents":[],"plain":"In 2010 muonic hydrogen (a muon orbiting a proton) gave a proton about 4% smaller than electron measurements did. New hydrogen measurements now agree with the small value, but some older hydrogen and scattering results still give the large value.","posed_since":"2010","precise":"Muonic hydrogen gives $r_p = 0.84087(39)\\,\\mathrm{fm}$ and the 2026 hydrogen 2S-6P measurement (Maisenbacher et al) gives $0.8406(15)\\,\\mathrm{fm}$, while the LKB 1S-3S (2018), H 2S-8D (2022) and Mainz electron scattering ($0.879\\,\\mathrm{fm}$) determinations favor larger values. Establish whether all hydrogen lines, combined with 1S-2S, yield a single $(R_{\\infty}, r_{p})$ pair consistent at the $10^{-12}$ level in $R_{\\infty}$.","problem_ref":"c8fb1852ecb4a0cd3bd5349236cf4d3c9cecb666980f4c0ebefc81995860a8f7","references":"","settled_by":"A global fit of all hydrogen and deuterium transition frequencies, with outliers explained by identified systematics, giving $R_{\\infty}$ and $r_p$ consistent with muonic values within uncertainties.","status_note":"The 2026 $2\\mathrm{S}-6\\mathrm{P}$ measurement agrees with muonic hydrogen and tests bound-state QED at 0.5 ppm; CODATA 2022 still reports a $2.8\\sigma$ difference between hydrogen-only and muonic data.","title":"Residual tensions in proton radius and Rydberg constant 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