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1

Quarks and antiquarks do annihilate, but generally in an indirect way, by forming a meson first. For example, in proton-antiproton annihilation, the strong interaction overwhelms the electromagnetic interaction, and the quarks and antiquarks rearrange into some number of pions. (The mean number of pions is about five; see this talk by Goldhaber for an ...


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I think people are usually referring to parity partners of hadrons and not of quarks as your title might suggest. The parity partner of the positive parity nucleon is for example the negative parity N(1535) (see for example PDG). Parity partner in this context refers to two hadrons which have identical quantum numbers except for parity.


2

So, unfortunately most of you said is wrong, and the reason is quite difficult to explain to a novice to (quantum) field theories. This is by no means your fault - it's a combination of ambiguous nomenclature and maybe simplistically intuitive analogies attempted in popular culture that have glorified what the "colour" charge means and represents ...


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There are some particles which are made of a specific composition of quarks and are always made of that composition. For example, as I believe you have heard of, protons and neutrons are always composed of $uud$ and $udd$, respectively. You can think of this intuitively in the following sense: if you could pick up a proton and look inside it to figure out ...


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Exactly as Mr. Zachos said, if the hadron has no valence strange or charm (or top or bottom) quarks, then the quark and antiquark distributions will be identical by the reasoning you gave (that they only ever come in pairs). For example, a proton has a valence structure of $|uud\rangle$. Therefore for the strange quark we'd have$f_{s/P}(x)=f_{\bar s / P}(x)$,...


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Recall that inertial mass was shown to be equal to gravitational mass experimentally and for a long time this was thought of as a coincidence mainly because no-one could think of a better explanation. Then of course Einstein discovered his equivalence principle and the rest is history. Likewise, this may pin to something more basic, the question is what. ...


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It's just a coincidence. Note that the original flavour $SU(3)$ permuting up, down and strange is broken by the differing masses, and the less badly broken $SU(2)$ is the isospin group. Both are thus approximate global symmetries. If you're fine with broken symmetries, you could add charm, bottom and top to go to $SU(6)$. However, the breaking gets ...


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The fundamental reason is experimental observations. The quark model was slowly built up over the years and the resonances measured in lab gave the surprising symmetries of the SU(3) group. In general, the number of basic constituents defines the dimension n of SU(n). (As an example SU(2) for neutron and proton in nuclear physics, fitted the data). For the ...


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