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Effective 4-quark Langrangian: ’t Hooft effective four-quark Lagrangianis “flavor nondiagonal,” leading to processes u→u(dd-)and d->d(uu-). In a way, the effect is also due to the Pauli exclusion principle, but at a different level. Topological tunneling events, known as instantons, create fields so strong that they fix the color and spin states of participating quarks uniquely. Instead of six possibilities, there remains only one, thus a complete blocking. Since the proton has two valence u quarks and only one valence d quark, that mechanism would suggest that the ratio of anti d to anti u is 2 rather than 1.
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