Credit: KOU Wei Researchers have recently extracted the proton mass radius from the experimental data. A research group at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) presented an analysis of the proton mass radius in Physical Review D on May 11. The proton mass radius is determined to be 0.67 ± 0.03 femtometers, which is obviously smaller than the charge radius of the proton. In the Standard Model, the proton is a composite particle made of quarks and gluons and it has a non-zero size. The radius of the proton is a global and fundamental property of the proton. It is related to the color confinement radius -- a property governed by quantum chromodynamics (QCD).