Proton radius puzzle
Why did early 2010s measurements of the proton's charge radius using muonic hydrogen come out about 4% smaller than the previously accepted value from electron-based measurements, and which value is correct?
History and references
Settled in 2019 by Multiple independent electron-scattering and spectroscopy experiments (e.g. Bezginov et al., Xiong et al.) converging on the smaller radius.
In 2010, Pohl et al. measured the proton's charge radius using muonic hydrogen and got a value about 4% smaller than the established electron-based value, an 8-sigma discrepancy that raised hopes of new physics. Through the 2010s a series of new experiments — improved electron-scattering (PRad, 2019) and new electron-hydrogen spectroscopy (Bezginov et al., 2019) — both landed on the smaller radius, resolving the puzzle in favor of the muonic measurement rather than exotic new physics. The Particle Data Group's 2025 consensus value is 0.8409(4) fm.
A tiny, stubborn 4% disagreement about the size of the humble proton briefly looked like it might crack open the Standard Model, before careful experimentation quietly closed the case.
Explanations that circulated
- That the puzzle revealed new physics beyond the Standard Model (e.g. muon-specific forces) — this was a live hypothesis for several years but the resolution instead came from improved, consistent measurements converging on the smaller radius
References
- A small proton charge radius from an electron-proton scattering experiment, W. Xiong et al. (PRad Collaboration), Nature, 2019
- A measurement of the atomic hydrogen Lamb shift and the proton charge radius, N. Bezginov et al., Science, 2019
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