The Proton Radius Puzzle

Abstract

The proton size, specifically its charge radius, was thought known to about 1% accuracy. Now a new method probing the proton with muons instead of electrons finds a radius about 4% smaller, and to boot gives an uncertainty limit of about 0.1%. We review the different measurements, some of the calculations that underlie them, some of the suggestions that have been made to resolve the conflict, and give a brief overview new related experimental initiatives. At present, however, the resolution to the problem remains unknown.

Accepted invited review for Progress in Particle and Nuclear Physics. 28 pages, 15 figures

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• Expanding Nuclear Physics Horizons with the Gamma Factory
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• Lepton-Proton Two-Photon Exchange in Chiral Perturbation Theory
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