What is the reason that the color properties we call red, green and blue have become tied to quarks, while what we call anti-red, anti-green and anti-blue has become tied to anti-quarks? Do note that I am NOT refering to the names (you can call any color value anything you want, after all), but the actual properties! Whatever the color Red really is, could that property have been tied to anti-quarks instead of quarks during the formation of the big bang? Or is there something special about the property we call Red that means it could only, ever, have become tied to quarks, and not anti-quarks??
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$\begingroup$ I'm moving this discussion to chat to keep a record of it. $\endgroup$– Emilio PisantyCommented May 31, 2016 at 19:21
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2 Answers
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(Do quarks have) color charges because of something?
Boris Struminsky is though to be one of the first to postulate an additional quantum number for quarks, allowing for the $\Omega^-$ hyperion (comprising three strange quarks) to exist without violating the Pauli exclusion principle. Color charge was, hence, introduced as solution to this problem.
Experimental evidence from $e^+e^-$ annihilation shows there to be three color charges.
Can quarks have anti-colors?
A basic tenet of QCD is that quarks must combine to be colorless. If quarks could gain anti-color we would expect to see quark-quark pairs but we don't.
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$\begingroup$ "If quarks could gain anti-color we would expect to see quark-quark pairs but we don't." - The idea was not that both would co-exist, but that roles would be swapped. What I am wondering is if what we call "anti-color" is unequivably tied to specifically what we call "anti-quarks", or if the match could have been the other way around, i.e. quarks had what we now call the "anti-colors", and anti-quarks had the "colors"? $\endgroup$ Commented May 31, 2016 at 18:22
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2$\begingroup$ The nomenclature of color is entirely arbitrary. Considering their symmetry nothing stopping us switching the terms "color" and "anti-color" or naming them something else entirely, but the physical properties labeled by these terms are distinct but symmetric. In the same way the way we label charge can sometimes cause confusion (physics.stackexchange.com/questions/17109/…) with symmetries the way we label is arbitrary. $\endgroup$– thodicCommented May 31, 2016 at 18:37
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$\begingroup$ True, and I edited the question phrasing to note that I am not talking about terminology, but whether the things we call "anti-colors" could only ever have teamed up with what we call "anti-quarks", or if the teams could have been switched, so that what we call "anti-color" was teamed with quarks, and vice versa. It's not a terminology question, although it can be misinterpreted as one. And yes, the electric charge terminology could be said to parallel that. $\endgroup$ Commented May 31, 2016 at 18:55
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$\begingroup$ @OhAuth See updated question. Also 'tenant' is presumably 'tenet'. $\endgroup$ Commented May 31, 2016 at 19:23
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If you take some quark flavors to have color charge, $\psi_f\to U\psi_f$ with $U\in SU(3)$, and some others to have anti-charge, $\psi_g\to U^*\psi_g$, then the lagrangian $$ {\cal L}=\sum_{h=f,g} \bar\psi_h i\gamma^\mu D_\mu \psi_h $$ will not be gauge invariant, because $D\to UDU^\dagger$. In other words, this theory violates conservation of color.
