The Proton Radius Puzzle

TL;DR

The proton radius puzzle involves conflicting measurements of the proton's charge radius using different methods, with recent muon-based results indicating a smaller size and higher precision, but the discrepancy remains unresolved.

Contribution

This paper reviews existing measurements, underlying calculations, proposed resolutions, and upcoming experimental efforts related to the proton radius puzzle.

Findings

Muon-based measurements suggest a smaller proton radius by about 4%.

Current uncertainties are around 0.1%, but the discrepancy remains unresolved.

Multiple hypotheses and new experiments are being explored to resolve the conflict.

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.