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I too see this often, @QMechanic. See, for example, Griffiths, Introduction to Elementary Particles, pg 47. By this they mean that the time it would take for signals to cross the 'length' of a typical hardon is much longer than the time it takes for a top (truth!) quark to decay. Since we assume cuasility, we cannot have an interaction which propagats ...

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A tetraquark (and pentaquark) had been previously predicted as extensions of known mesons (and baryons, respectively). There is no reason why they cannot be colour neutral. e.g. in RBG the quarks could be R 255 G 0 B 0 R 0 G 255 B 0 R 0 G 0 B 128 R 0 G 0 B 127

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There are three flavours of quarks in the fundamental $3$ representation of $SU(3)$, the QCD gauge group. Their antiparticles are in the conjugate representation $\bar3$ or $3^\star$. QCD is confining; the quarks form bound, colorless states, which are singlets in $SU(3)$. Mesons are $q\bar q$. The general tensor $3\times\bar 3$ can be decomposed into ...

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Since electrons don't interact through the strong interaction, an electron-quark "atom" is on the face of it the same as an electron-proton atom. (Except maybe weak interaction decays, I'm not entirely sure.) However: a free quark has never been observed in experiments, and it is widely believed - but not proved - that the theory of strong interactions does ...

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