Gluon spin and quark confinement

[joly]

Gluons are spin 1 particles so the Strong Force can both attract and repulse. The constituent partons of a meson are a quark and an antiquark so they must carry a given color and its anticolor, respectively, in order that there is no net color carried by the parton. In that case, the force between the quark and antiquark must be always attractive since it involves opposite type of charges. Is that reasoning correct?

If so, how does this work for a baryon, which contains 3 constituent quarks of 3 different colors? Could we have both attractive and repulsive components in the (overall attractive) confinement force, or is QCD also always attractive in that case?

 

[mfb]

Hadrons are not as easy as N quarks sitting around. The valence quarks are important, but the whole QCD mess is important as well.

The valence quarks in a hadron feel an effective attraction to the other two valence quarks as well. "Blue+red" combined is very similar to "anti-green", for example. But don't take that too literally.

 

[Garlic]

"Blue+red" combined is very similar to "anti-green"

Doesn't blue+red combined exactly give anti-green? (I'm asking this because you wrote similar)

 

[joly]

Thank you. I was thinking of the heavy (static) quark, quenched, approximation that is e.g. studied by lattice QCD. This is a simple case to picture the confinement mechanism, although as you say it does not contains the full story: the QCD string appears to break down when light quark pair-creation is allowed. Nevertheless, it seems educative to consider the static quenched approximation even if it is not fully QCD. In that case, I suppose that the valence quark picture should be reasonable. If so, does the reasoning based on the color-anticolor configuration of the q-qbar pair leading to an always attractive force makes sense?

For the baryons, blue+red=anti-green seems to be a good answer. It reminds of the quark-diquark picture.

 

[mfb]

Doesn't blue+red combined exactly give anti-green? (I'm asking this because you wrote similar)

If you add the colors, yes, but you have two different valence quarks with those colors, not a single anti-green valence quark.

If so, does the reasoning based on the color-anticolor configuration of the q-qbar pair leading to an always attractive force makes sense?

Yes.