The proton is not a point particle; nor is it an unvarying composite. Inside the proton are three valence quarks—two up quarks and one down quark—accompanied by massless gluons and a sea of other quarks that flit in and out of existence. Particle physicists characterize the proton’s structure with so-called form factors. The two most significant of those relate to the charge- and magnetic-moment distributions in the proton. Their determination is important not just for describing the internal structure of the proton but also for understanding quark–gluon interactions within it and for interpreting experimental determinations of, for example, the proton radius.
The two form factors can be extracted from measurements of elastic scattering of unpolarized electrons off protons. Surprisingly, polarized-electron scattering experiments conducted about 15 years ago at the Thomas Jefferson National Accelerator Facility contradicted the apparently well-established results obtained earlier with unpolarized electrons.1 Theorists soon proposed a reason...
