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Pions consist of quark and antiquark and strong force keeps them together. So color charge and anticolor charge attracts each other. But in proton we have 3 quarks and they also attract each other. It seems counter-intuitive that regardless of color charge sign we have pulling force.

It becomes even more strange when we include color into consideration. In proton we have red, green and blue quark. Considering only one kind of charge, let it be red, we see that only one quark is charged red and two others are not charged. Yet they attracts.

How it is possible that all common color charge combination results in attractive force? Is it possible at all that two quarks repel each other?

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    $\begingroup$ Possible duplicate: physics.stackexchange.com/questions/48569/…. In particular, the accepted answer there purports to show that red-anti-blue interactions are repulsive whilst colorless combinations are always attractive. $\endgroup$
    – jacob1729
    Commented Nov 1, 2019 at 20:38
  • $\begingroup$ Not exactly duplicate but you are right - this article explains color charge interactions. I have to read it in more detail but I agree that my question could be treated as duplicate. Thank you for finding the article so fast! $\endgroup$
    – robsosno
    Commented Nov 1, 2019 at 21:00

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Thanks to jacob1729 who shed the light on the subject. I'm not a professional so I don't read books and articles about gauge theory. And popular articles does not explain why both quark-antiquark pair attracts and 3 quarks attracts each other too.

What I've understood is following: color charges: red, green, blue are not independent. There are forces between all kinds of these charges but with different coupling constants. So red-antired charges attract each other but red-red repels each other as intuition indicates. And for red-green-blue charges coupling is such that they atracts each other.

The paradox is solved here by having more general definition of charge. I'll try to read more about this. I've just needed staring point.

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The attraction and repulsion is based on the mediator. If the mediator boson spin is odd, like the gluon, you get like charges (color) repel, and opposite charges attract.

https://physics.stackexchange.com/a/119873/132371

Now this strong force is very distance dependent, because it is not always attractive. I actually asked a question about this.

The strong force does pull quarks together, but it also gets weaker as the quarks get closer (i.e. it acts sort of like a spring), in a phenomenon known as "asymptotic freedom." In this way, the strong force is very different than electromagnetism, where the force gets stronger if the charges are closer together. As such, there's no reason to expect that quarks which are placed close together will immediately annihilate, as there's just not a lot of force on them in the first place.

https://physics.stackexchange.com/a/396054/132371

Quarks don't exist as free charged objects on which we could take the classical limit and consider "forces" on them. They are confined, and occur only as constituents of bound states. In quantum mechanics, it doesn't make sense to ask whether the constituents of a bound state "repel" or "attract" each other.

https://physics.stackexchange.com/a/254307/132371

So basically we cannot experimentally tell whether two quarks would repel if they had same color, but theoretically they would (because of the odd spin of the gluon).

Quarks of different color experience an attractive force between them; whereas quarks of a like color repel each other.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/quark

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  • $\begingroup$ why the downvote? $\endgroup$
    – Árpád Szendrei
    Commented Nov 2, 2019 at 16:02
  • $\begingroup$ "If the mediator boson spin is odd, like the gluon, you get like charges repel" . So why gluons in neutron (or proton) bind quarks? The quarks have like charges so they should repel. $\endgroup$
    – robsosno
    Commented Nov 2, 2019 at 19:15
  • $\begingroup$ @robsosno inside a neutron or proton, quarks do not have like color charges. One quark is red, other green, blue. Those attract. $\endgroup$
    – Árpád Szendrei
    Commented Nov 2, 2019 at 19:17
  • $\begingroup$ Ok, then you have one red charge and two other neutral quarks in regards to red charge. So they do not have like charges neither opposite. What is the source of force then? $\endgroup$
    – robsosno
    Commented Nov 2, 2019 at 19:23
  • $\begingroup$ @robsosno you have a red, green and blue. red and green attract, blue and green attract, red and blue attract. Opposite means not same in color. This is because you get confused when they say it is like EM. In EM there is only one opposite. In color, there are two opposites. $\endgroup$
    – Árpád Szendrei
    Commented Nov 2, 2019 at 19:38

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