Proton's Mass Radius Shorter Than Charge Radius

Experimental work has created a value for the proton's mass radius, which describes the distribution of mass within the particle. The proton's mass primarily depends on its gluons, which are difficult to detect. By measuring the production of J/ψ mesons, it's possible to determine something called the gluonic gravitational form factors, which describes where the mass sits in the proton. It turns out the value is significantly different from the proton's charge radius.