Why the Proton 'Lost Weight' and Then 'Gained It Back' express
To uncover the proton's true size, physicists swapped the electron in a hydrogen atom for its heavy copy — the muon. Acting like a magnifying glass with thousandfold power, the muon gets right up against the nucleus and 'probes' the proton with unprecedented precision. Spectral analysis revealed the proton to be roughly 4% smaller than earlier experiments with ordinary electrons suggested.
This challenged the Standard Model and its principle of lepton universality — the notion that all light particles follow the same electric laws. Two different magnifying glasses seemed to paint conflicting pictures of a single object.
But re-examining the old data exposed a flaw. The electron-based 'magnifying glass' harbored an unnoticed error: it slightly distorted the image. After recalibration, both glasses agreed on the same size. The puzzle dissolved; physics stood firm.
Original abstract
as written by the authors arXiv:2604.15546
Pohl et al. measured the energy difference between the 2P and 2S states of muonic hydrogen and used it to determine a precise value of the proton radius. The result disagreed significantly from values extracted from electronic hydrogen and elastic electron-proton scattering. This discrepancy was exciting because it indicated a breakdown of Coulomb's law. In more technical terms, the discrepancy indicated that a fundamental property of the Standard Model, known as lepton universality, could be violated. This chapter explains the origins, meaning and significance of the puzzle. A resolution, based on very recent experiments, is stated. The proton radius puzzle is no more.
The muon decays in 2 microseconds. Light travels only 600 meters in that time. Yet it's enough for muonic hydrogen to yield the most precise proton size measurement in history.