Resizing the proton radius

What is the value of the proton charge radius? There is no easy answer to this seemingly simple question, as illustrated by an enduring debate that originated in 2010 and sometimes referred to as the “proton radius puzzle”. When a spectroscopic measurement of muonic hydrogen reported a value for the proton charge radius smaller than that previously found through electron–proton scattering experiments and hydrogen spectroscopic measurements, it became clear that it would be necessary to re-examine past data, improve the theoretical framework and carry out new measurements with higher precision and a less stringent model dependence. Here, the PRad collaboration at Jefferson Lab reports the results of their latest electron-scattering experiments for the determination of the proton charge radius. They find a value in agreement with the muonic hydrogen investigations, thus supporting a radius smaller than that previously obtained in a study that also used high precision electron–proton scattering.