The proton’s charge and magnetization

The proton’s charge and magnetization have different spatial distributions, new experiments have found. It has long been known that the proton’s electric charge and magnetic moment were each spread throughout the particle. The details, however, have remained unclear as to how the three quarks that make up the proton—and the gluons that hold those quarks together—interact to give the proton its electric and magnetic properties. In recent Jefferson Lab experiments, physicists fired an intense beam of polarized electrons at a target of liquid hydrogen and measured the degree of polarization transferred to the proton. The electrons’ energies spanned 1000–5700 MeV and thereby probed features within the proton on various spatial resolutions, down to 10% of the proton’s radius. The measurements showed that the electric charge is distributed over a larger volume in the proton than is the magnetization. Thus, the magnetic moment in all likelihood arises from a combination of the intrinsic spins of the quarks and gluons and the motions of the electrically charged quarks. Olivier Gayou, a graduate student at the College of William and Mary, reported the experimental results at the April meeting of the American Physical Society.