# Gluons and quarks and spin

1. Aug 25, 2004

### kurious

Under what circumstances could the colour force ( or the strong residual force) become repulsive between quarks? I've heard that changing the spin of a force carrier can in principle make a force repulsive when it was previously attractive.If gluons exchanged between quarks became fermions with spin 1/2 would this cause a repulsive force between quarks?

2. Aug 25, 2004

### zefram_c

The color force can be repulsive as it stands. Let's say you have two quarks that do not form a color singlet (eg, suppose they are both red quarks). Then the force between them is repulsive, and they cannot form a meson.
I am not aware of any theories involving spin-1/2 mediators or how they would look like. Even integer spin (0,2,etc) carriers always convey an attractive force, and odd integer spin carriers can convey either attractive or repulsive forces; it depends on the circumstances.

3. Aug 26, 2004

### marlon

Gluons are a quark-antiquark condensate and thus always have integer spin. They are bosons.

The strong force becomes repulsive when two identical quarks interact. By identical one means : the same quantumnumbers, just as zefram_c pointed out.

regards
marlon

4. Aug 26, 2004

### humanino

Marlon, it is not the first time I read in your posts :
Why do you seem to refuse gluons as particles by themselves
The gluon is analogous to the photon field when it comes to question "who ordered that". The gluon is the gauge field.

Now, of course, the double-lines formalism which allow one to keep track of the color indices tend to suggest the gluon are quark-antiquark pairs. Do you take that seriously

5. Aug 26, 2004

### marlon

Your first point is completely correct. Maybe my explanation with this quark-antiquarkpair thing is a bit confusing sinve the gluons do not have restmass.

It is because of the way they interact that I state this. i do take that seriously...

regards
marlon

6. Aug 26, 2004

### marlon

Gluons are spin 1 particles, that is why I make this poor analogy.

i won't do this again, since it is indeed confusing...

thanks for the correction humanino

7. Aug 26, 2004

### humanino

I was not really trying to correct you. I mean, if someone wants to question the reallity of those particles, I could not argue easily with him. There is no doubt that the electron is real. But quarks and gluons are never free. So, your opinion on what they really are can only be judged with respect to the computation efficiency of your concepts. The double line formalism is indeed very efficient !

8. Aug 27, 2004

### jtolliver

If a quark were to emit a spin-1/2 fermion, by conservation of spin it would change into someother type of particle(a boson with either spin 0 or spin 1). The color charge would also have to go somewhere. If the spin-1/2 particle has color charge, it is probably just a quark. The most well known example of this is pair annihilation; a quark emits a virtual quark and in the process becomes either a photon or a gluon, then the virtual quark is absorbed by an antiquark, which also becomes a photon or a gluon. If the spin-1/2 particle didn't have color charge, the boson that the quark becomes does(AFAIK no bosons with color charge have been experimentally observed).

Any force caused by the interchange of spin-1/2 particles must be an inverse cube force(this can be shown from the fact that the fourier transform of the propagator has units of length, so after integrating over all 4 momentum components you get units of length-4*length = length-3).

9. Aug 28, 2004

### kurious

jtolliver:
If the spin-1/2 particle didn't have color charge, the boson that the quark becomes does(AFAIK no bosons with color charge have been experimentally observed).

Kurious:
What if the colour field is carried in the boson as vectors like electric and magnetic field vectors? Would those vectors be observable in some way?