The Lamb shift in muonic hydrogen and the proton radius puzzle

Abstract

The recent measurement of the Lamb shift in muonic hydrogen resulted in a tenfold improved value of the rms charge radius of the proton. The value is, however, 7 standard deviations discrepant from the world average of this quantitiy which is obtained from elastic electron-proton scattering and precision spectroscopy of hydrogen and deuterium. New input from both theory and experiment is needed to resolve this so-called “proton radius puzzle”.