Do the colors of quarks really exist or are they just a convention in QCD?

[abdullahbameh]

the colors of the quarks!

can anybody tell me
if the colors of the quarks does really exist (( green , blue , red)) ((anti green, anti blue, anti red ))) or it is just a way to avoid the exclusion principle??
and they are fermions means they can not exist in the same state and have half integral spin.

 

[humanino]

The number of colors can be measured in different ways. On way, you collide electrons and positrons, and you look at the ratio of the probability for a quark-antiquark pair, to the probability to get (say) a muon-antimuon pair (see Drell-Yan process as well, but this is more complicated for this purpose). This is sensitive to both the number of generations (flavor) involved, which depends on energy, and the number of colors. This definitely rules out any scenario except 3 colors. Another way, the neutral pion lifetime, is quite unambiguous as well. Then, the physics of jets as come to such a richness that I cannot summarize now. For instance, the scaling violations in jet production, we can even compare quark and gluon jets, everything points to SU(3).

 

[tiny-tim]

Hi humanino!

But do the colours exist as discrete characteristics, in the same way that charge does?

Charge can be either + or -, but not, for example, halfway between + and -.

Can a quark have a colour, for example, halfway between red and green?

 

[Vanadium 50]

This is a complicated question. The problem is that the symmetry group of QCD, SU(3) is not the same as the symmetry group of electromagnetism, and a lot of the properties we think of as covered by the word "charge" really only apply to U(1) theories.

One thing that happens is that if you do a calculation in the red-blue-green basis, and I do the same calculation in a rotated basis, say purple-aqua-brown, we will agree on the outcome. So the choice of what a "red" quark is entirely by convention.

Another is that one would think a gluon, carries two units of charge. That's approximately right, but you don't just get to say "red makes one and anti-blue makes one more" - you have to use the T matrices, and sometimes instead of 2 you get numbers like maybe 15/8. This is a direct consequence of the more complex groupn structure.