Extraction of the proton charge radius from experiments

TL;DR

This paper reviews various methods used to extract the proton charge radius from experiments, highlighting discrepancies and the ongoing efforts to resolve the proton radius puzzle.

Contribution

It provides a comprehensive review of experimental and theoretical approaches to determine the proton charge radius and discusses the differences in results from various methods.

Findings

Muonic hydrogen spectroscopy yields a smaller radius than electron scattering.

Discrepancies between measurement methods constitute the proton radius puzzle.

Numerous approaches have been attempted to resolve these differences.

Abstract

Static properties of hadrons such as their radii and other moments of the electric and magnetic distributions can only be extracted using theoretical methods and not directly measured from experiments. As a result, discrepancies between the extracted values from different precision measurements can exist. The proton charge radius, $r_p$, which is either extracted from electron proton elastic scattering data or from hydrogen atom spectroscopy seems to be no exception. The value $r_p = 0.84087(39)$ fm extracted from muonic hydrogen spectroscopy is about 4% smaller than that obtained from electron proton scattering or standard hydrogen spectroscopy. The resolution of this so called proton radius puzzle has been attempted in many different ways over the past six years. The present article reviews these attempts with a focus on the methods of extracting the radius.