The Proton Radius Puzzle

TL;DR

The proton radius puzzle involves a significant discrepancy in measurements of proton size from muonic hydrogen versus electronic methods, challenging fundamental physics assumptions, but recent experiments suggest it has been resolved.

Contribution

This paper explains the origins, significance, and recent resolution of the proton radius puzzle, a major discrepancy in fundamental physics measurements.

Findings

Recent experiments resolve the proton radius discrepancy.

The puzzle indicated potential violation of lepton universality.

The discrepancy no longer persists after recent measurements.

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.