The Proton Radius Puzzle
Abstract
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.
Cite
@article{arxiv.2604.15546,
title = {The Proton Radius Puzzle},
author = {Gerald A. Miller},
journal= {arXiv preprint arXiv:2604.15546},
year = {2026}
}
Comments
12 pages, 7 figures. Prepared for Encyclopedia of Nuclear Physics, references added and small corrections