Proton Charge Radius from Lepton Scattering

TL;DR

This paper reviews the measurement of the proton charge radius via lepton scattering, discussing recent developments, ongoing puzzles, and future experimental directions in resolving discrepancies between different measurement methods.

Contribution

It provides a comprehensive overview of recent theoretical and experimental advances in lepton scattering measurements of the proton charge radius and discusses future research prospects.

Findings

Recent measurements highlight discrepancies in $r_{E}^{p}$ values.

Advances in experimental techniques improve measurement precision.

Ongoing theoretical efforts aim to resolve measurement inconsistencies.

Abstract

Protons are bound states of the strong interaction governed by Quantum Chromodynamics (QCD). Its charge radius ($r_{E}^{p}$) is an important quantity as it characterizes the spatial distribution of the proton's charge, which is carried by the quarks. On the other hand, the proton charge radius is an essential physical input for the bound-state Quantum Electrodynamic (QED) calculations for the hydrogen atomic energy levels. Nevertheless, the large discrepancy between $r_{E}^{p}$ measurements from muonic hydrogen spectroscopy, and those from $ep$ elastic scattering and ordinary hydrogen spectroscopy, have been puzzling physicists for over a decade. Tremendous efforts, in both theoretical and experimental sides, have been dedicated to providing various insights into this puzzle, yet certain issues still remain unresolved, particularly in the field of lepton scatterings. This review will focus on $r_{E}^{p}$ measurements using lepton scatterings, the recent theoretical and experimental developments in this field, as well as future experiments using this technique.