The quest for the proton charge radius

TL;DR

This paper reviews the history and ongoing efforts to precisely determine the proton charge radius, highlighting the proton radius puzzle caused by discrepancies between different measurement methods and emphasizing the need for future research.

Contribution

It provides a comprehensive overview of the experimental and theoretical developments in measuring the proton charge radius and discusses the unresolved issues in the field.

Findings

Discrepancy between electronic and muonic hydrogen measurements

Decade-long research activity addressing the proton radius puzzle

Need for further precise measurements to resolve the puzzle

Abstract

A slight anomaly in optical spectra of the hydrogen atom led Willis E. Lamb to the search for the proton size. As a result, he found the shift of the 2S1/2 level, the first experimental demonstration of quantum electrodynamics. In return, a modern test of QED yielded a new value of the charge radius of the proton. This sounds like Baron Muenchausens tale: to pull oneself out from the marsh by seizing his own hair. An independent method was necessary. Muonic hydrogen spectroscopy came to the aid. However, the high-precision result significantly differed from the previous, electronic, values: this is the proton radius puzzle. This puzzle produced a decade-long activity both in experimental work and in theory. Even if the puzzle seems to be solved, the precise determination of the proton charge radius requires further efforts in the future.