What is an electromagnetic radius?

The electromagnetic radius is a measure of the spatial distribution of an electrically charged particle's charge, often used in the context of protons and nuclei. It is derived from the proton's form factor, which describes how the charge density of the particle varies with distance. This radius reflects the size of the charge distribution rather than the physical size of the particle itself. Typically, the electromagnetic radius provides insights into the internal structure of particles in nuclear and particle physics.