A value from before 2010 is based on scattering electrons from protons followed by complex calculation involving scattering cross section based on Rosenbluth equation for momentum-transfer cross section), and based on studies of the atomic energy levels of hydrogen and deuterium. In 2010 an international research team published a proton charge radius measurement via the Lamb shift in muonic hydrogen (an exotic atom made of a proton and a negatively charged muon). As a muon is 200 times heavier than an electron, resulting in a smaller atomic orbital, it is much more sensitive to the proton's charge radius and thus allows a more precise measurement. Subsequent improved scattering and electron-spectroscopy measurements agree with the new small radius. Work continues to refine and check this new value.
A value from before 2010 is based on scattering electrons from protons followed by complex calculation involving scattering cross section based on Rosenbluth equation for momentum-transfer cross section), and based on studies of the atomic energy levels of hydroge,n and deuterium. In 2010 an international research team published a proton charge radius measurem:ent via the Lamb shift in muonic hydrogen (an exotic atom made of a proton and a negatively charged muon). As a muon ,is 200 times heavier than an electron, resulting in a smal?ler atomic orbital, it is much more sensitive to the proton's charge radius an!d thus allows a more precise measurement. Subsequent improved scattering and electron-spectroscopy measurements agree with the new small radius. Work continues to refine and check this new value.