I saw this in wiki https://en.wikipedia.org/wiki/Antiproton#Modern_experiments_and_applications "the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the largest fraction of the proton or antiproton's momentum." However valence quarks only account for a very small fraction of the proton's mass so how do they carry most of its momentum? Or is it virtual particles that does that?

  • $\begingroup$ Eeeek! you are misreading the (somewhat ambiguous) sentence! "Largest fraction" here means Feynman x, so 1/3ish of a huge kinetic momentum, and definitely not the "fraction of mass" popular in recent estimates of the energy-momentum tensor of hadrons and misrepresented virulently by the popular science press... $\endgroup$ May 22, 2019 at 15:24
  • $\begingroup$ @Cosmas Zachos sounds like that is an answer not just a comment $\endgroup$
    – KF Gauss
    May 22, 2019 at 21:34
  • $\begingroup$ So the rest of the momentum came from the energy inside the proton? I'm not entirely sure what u meant there. Are there other things that can contribute to its momentum? $\endgroup$ May 23, 2019 at 2:20
  • $\begingroup$ The rest of the momentum comes from gluons and from sea quarks. $\endgroup$ May 23, 2019 at 2:52

1 Answer 1


Momentum, not mass. The sentence you are misreading refers to highly energetic protons with kinetic energies dozens of times higher than the mass of the nucleon or antinucleon in question, the realm of perturbative QCD. The valence partons involved carry a fraction x of the momentum of the struck nucleon —see the formal definition in the PDG review — of the order of 1/3, as visible in the green and blue distributions of the WP article figure. By contrast, "sea" quarks carry smaller fractions of the momentum. Gluons carry collectively about a half.

These are completely different fractions and entities than the accounting of operators expectations fashionable in computing nucleon masses in highly nonperturbative QCD via lattice simulations, and reported rather ineptly in the popular science press: veritable recipes for misconceptions. The quark operators in all of these accountings are definitely not valence quarks, they are ultra-light current (fundamental) quarks, and the small fraction is an operator corresponding to just their mass terms ("condensate"), not their over-all "energy" and "anomaly" contribution, the bulk of the "heft".

The methods, culture, significance and mental pictures involved in each of the above paragraphs are so disparate that calling them apples and oranges is flattering them (!) with conceptual connections that simply are not there. A particle physics student is warned early and often to never put these two domains in his visual field or mental buffer simultaneously, as ineffable grief cannot fail to ensue...


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.