The Puzzling Asymmetry of the Proton Sea

Students of particle physics learn early that the proton is made up of three quarks: two up and one down. But that simple picture doesn’t tell the whole story. The quark masses, at a few MeV each, contribute just 1% of the proton’s total mass of 938 MeV. The rest comes from the quarks’ kinetic energy, the gluon field that holds them together, and a fluctuating sea of fleeting pairs of quarks and antiquarks.