The size of the proton

Abstract

In this work, the atomic energy levels for hydrogen-like atoms, in particular, for muonic hydrogen are described. We discuss why this exotic atom is a good candidate to help measure the proton radius. Such experiments with muonic hydrogen have been performed very recently and have shown a “shrinkage” of the proton size in disagreement with previous measurements. This so-called “proton radius puzzle” is still under debate but we summarize some possible explanations that have been proposed