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Title: Residual tensions in proton radius and Rydberg constant determinations
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.
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_inf, r_p) pair consistent at the 1e-12 level in R_inf.
Settled by: A global fit of all hydrogen and deuterium transition frequencies, with outliers explained by identified systematics, giving R_inf and $r_p$ consistent with muonic values within uncertainties.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: partially-resolved
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.
Posed since: 2010