The quest for the proton charge radius

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 2S 1/2 level, the first experimental demonstration of quantum electrodynamics (QED). In return, a modern test of QED yielded a new value of the charge radius of the proton. This sounds like Baron Münchausen's 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 (was?) the proton radius puzzle (2010-2020?). 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.