Proton Electric Charge Radius from Lepton Scattering

A proton is a bound state of a strong interaction, governed by Quantum Chromodynamics (QCD). The electric charge radius of a proton, denoted by r(E)p, characterizes the spatial distribution of its electric charge carried by the quarks. It is an important input for bound-state Quantum Electrodynamic (QED) calculations of the hydrogen atomic energy levels. However, physicists have been puzzled by the large discrepancy between r(E)p measurements from muonic hydrogen spectroscopy and those from ep elastic scattering and ordinary hydrogen spectroscopy for over a decade. Tremendous efforts, both theoretical and experimental, have been dedicated to providing various insights into this puzzle, but certain issues still remain unresolved, particularly in the field of lepton scatterings. This review will focus on lepton-scattering measurements of r(E)p, recent theoretical and experimental developments in this field, as well as future experiments using this technique.

Automated summary

A proton is a bound state of strong interaction governed by Quantum Chromodynamics (QCD), and its electric charge radius is important for Quantum Electrodynamic (QED) calculations. However, there has been a discrepancy between measurements of proton charge radius from different methods. This review focuses on lepton-scattering measurements of proton charge radius, recent theoretical and experimental advancements, and future experiments in this field.