We have known since 1968 that protons are not elementary particles and are comprised of three persistent quarks: two up quarks with charge +⅔e, and one down quark with charge -⅓e, where e is the charge of the electron (which is an elementary particle).
But in a brilliant illustration of E = mc2, we now know that very little of the proton’s 938 MeV rest mass comes from the rest mass of its component up and down quarks.
The mass composition of a proton is:
½% up quark rest mass (¼% × 2)
½% down quark rest mass
8% sea quarks (virtual quark-antiquark pairs created by the strong nuclear force)
23% quark-gluon interaction energy (the trace anomaly)
32% kinetic energy of the up and down quarks
36% momentum energy of the massless gluons that hold the up and down quarks together within the proton
We thus see that only 1% of the rest mass of a proton is provided by the rest mass of its three component valence quarks (2 up + 1 down), and the other 99% is interaction energy!
Even though protons are in the nucleus of every atom of matter in the universe, we still do not fully understand them. For example, there is the proton radius puzzle, the proton spin puzzle, and the important question of proton decay.
E. Conover. How the proton’s mass adds up. Science News. Vol. 194, December 22, 2018, p. 8.
E. Conover. There’s still a lot we don’t know about the proton. Science News. Vol. 191, April 29, 2017, p. 22.
Y.B. Yang et al. Proton mass decomposition from the QCD energy momentum tensor. Physical Review Letters. Vol. 121, November 23, 2018, p. 212001. doi: 10.1103/PhysRevLett.121.212001.