{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"f2a2b638f41adbd737ee6ed3269e8f6a3726c8a40a26fbee6926452f6b943f1d","created":"2026-10-03T07:18:07Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"594ec753562f061f1bbcd75b6daae6eb1102fcb754bc6707ddbb44a82034c068","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"nuc.proton-structure.charge-radius-residual","kind":"phenomenon","literature_status":"partially-resolved","n":"1","parents":[],"plain":"Muonic hydrogen (a proton orbited by a muon, a heavy cousin of the electron) and the most precise hydrogen measurements give a proton radius of about $0.84\\,\\mathrm{fm}$ ($1\\,\\mathrm{fm} = 1e-13\\,\\mathrm{cm}$). Electron-scattering experiments disagree with each other, one giving $0.83\\,\\mathrm{fm}$ and an older one $0.88\\,\\mathrm{fm}$, and the cause is not identified.","posed_since":"2019","precise":"The rms charge radius $r_{p} = 0.84087(39)\\, \\mathrm{fm}$ from the muonic hydrogen Lamb shift (2013) agrees with hydrogen $2\\mathrm{S}-6\\mathrm{P}$ spectroscopy (0.8406(15) fm, Maisenbacher et al., 2026) and with PRad low-momentum-transfer e-p scattering (0.831(14) fm, 2019), whereas Mainz A1 e-p scattering gives 0.879(8) fm (2010). Identify the cause of the scattering discrepancy: radiative corrections, cross-section normalization, or the functional form used to extrapolate $G_{E}(Q^{2})$ to $Q^{2} = 0$. The remaining spectroscopic outliers (LKB $1\\mathrm{S}-3\\mathrm{S}$, 0.877(13) fm, 2018; Colorado State $2\\mathrm{S}-8\\mathrm{D}$, about 0.86 fm, 2022) are a question of hydrogen line shapes and QED corrections and are treated as an atomic-physics problem. Answer: the identified systematic effect, shown by reanalysis or new data that bring the scattering values into agreement.","problem_ref":null,"references":"","settled_by":"Low-momentum-transfer e-p and mu-p scattering (PRad-II, MUSE, AMBER) analyzed with a common radiative-correction and fitting treatment, together with an independent reanalysis of the Mainz A1 data.","status_note":"The MPQ hydrogen 2S-6P measurement (Maisenbacher et al., Nature 2026, arXiv 2602.14980) confirmed the small radius; the origin of the larger Mainz A1 scattering value remained unexplained as of 2026.","title":"Disagreement between electron-scattering values of the proton charge 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