The so-called proton charge radius puzzle was one of the challenging problems in physics in the last decade. A significant (at the level of four standard deviations (4σ) difference between the values of the root-mean-square proton charge radius measured in normal and muonic hydrogen has kindled lively discussions among both experimentalists and theoreticians specializing in quantum electrodynamics. The problem becomes even more glaring (up to 7σ) if data on the scattering of electrons on protons are taken into account. We review various methods that enable measurement of the proton charge radius, analyze the origin of the disagreement, and present results of recent experiments that aim at resolving this puzzle.

Keywords: proton radius, hydrogen atom, muonic hydrogen, proton radius puzzle, Rydberg constant, single-photon spectroscopy, e-p scattering PACS: 06.20.Jr, 06.30.Ft, 12.20.Fv, 32.10.Fn, 32.30.Jc, 42.62.Fi (all) DOI: 10.3367/UFNe.2021.06.038986 URL: https://ufn.ru/en/articles/2021/10/d/ 000740826300010 2-s2.0-85123456182 Citation: Khabarova K Yu, Kolachevsky N N "Proton charge radius" Phys. Usp. 64 1038–1048 (2021) BibTexBibNote ® (generic)BibNote ® (RIS)MedlineRefWorks Received: 11th, February 2021, revised: 4th, June 2021, accepted: 6th, June 2021 Оригинал: Хабарова К Ю, Колачевский Н Н «Зарядовый радиус протона» УФН 191 1095–1106 (2021); DOI: 10.3367/UFNr.2021.06.038986