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Meson Cloud Model: At any given instant, the proton might really be a neutron (ddu) plus a positively charged pion (ud- ud-)—or another proton (uud) plus a neutral pion (an equal superposition of uu‾uu‾ and dd‾dd‾ uu-uu- and dd-dd-). This violates energy conservation but it is allowed, for a fleeting moment, by the Heisenberg uncertainty principle. By adding up the contributions from all the possible channels, the theorists can model the composition of the sea.
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Effective 4-quark Langrangian: ’t Hooft effective four-quark Lagrangianis “flavor nondiagonal,” leading to processes u→uu→u(d‾d-) u→u(d‾d) and d→d->d(u‾u)d→d(u‾u) but not to u→u(u‾u)u→u(u‾u) and d→d(d‾d)d→d(d‾d)u-). 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 d‾-/ = 2 u‾d‾/u‾- rather than 1.
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