# QCD Question

1. Sep 15, 2015

### Eli137

Someone please explain what the colors mean in QCD. Are different colors different quarks? Are different colors the same quark with different amounts of energy? Is it something else entirely? Does it apply to leptons?

2. Sep 15, 2015

### phyzguy

It's similar to electric charge. A particle can have either a positive or a negative electric charge, and if you combine two particles, one positive and one negative, the combination has no electric charge and is neutral. Color charge is conceptually similar, but more complicated. Instead of just positive and negative charges, there are really six types of color charge, red, anti-red, blue, anti-blue, green and anti-green. If you combine a color and an anti-color (say red and anti-red) the combination has no color charge. We say it is colorless, color-neutral, or 'white'. Also, if you combine all three (red, green and blue, or anti-red, anti-green and anti-blue), that combination is also colorless. All quarks carry color charge, and can come in all three colors. So an up quark can be either red, blue, or green, and an anti-up can be either anti-red, anti-blue, or anti-green. The strong force is so strong, that individually colored particles cannot be separated, so all strong interacting particles that we detect (called hadrons) are always colorless. There are two main types of hadrons. The first are mesons, which consist of one colored quark (red green or blue) and one colored anti-quark (anti-red, anti-blue, or anti-green), so that the combination is colorless. The second type are called baryons, which consist of three quarks, one of each color, so that they are also colorless.

All quarks carry color charge. Leptons do not carry color charge.

3. Sep 15, 2015

### Eli137

Thank you for that great answer, but I still have one question. How can an up quark be different from another up quark? Does this suggest that preons exist?

4. Sep 15, 2015

### phyzguy

There are three different types of up quark, one of each color. They are identical except for their color charge. I don't see that this requires the existence of preons.

5. Sep 15, 2015

### Eli137

I was thinking about it like chemistry, since that is easier for me to understand. Instead of quarks, I thought of Hydrogen atoms. Instead of color charge, I thought of electric charge. If one hydrogen ion is more negatively charged than another, this is due to it having more electrons. I therefore thought that a difference in color charge for the same type of quark would require different composition.

6. Sep 16, 2015

### Demystifier

If you want to think about it like chemistry, then perhaps color of the quark is the most similar to spin of the electron. As you might know, spin can be viewed as a row consisting of two numbers (a,b), e.g. (1/2,0) or (0,1/2). Likewise, color can be viewed as a row consisting of three numbers (A,B,C). Of course, quark also has spin, so quark can be more fully characterized by two rows (a,b) $\times$ (A,B,C).

Last edited: Sep 16, 2015
7. Sep 16, 2015

### Eli137

So can a red quark become a blue or green quark?

8. Sep 16, 2015

### ddd123

Different colors don't mean more or less than a quantity (energy, color charge, whatever). The three colors are orthogonal properties. Though an anti-color is the negative of that specific color number:

9. Sep 16, 2015

### Demystifier

Yes.

10. Sep 16, 2015

### Eli137

W
What makes a quark change color charge?

11. Sep 16, 2015

### Demystifier

Interaction with another quark, via a gluon. The total color charge of the whole system cannot change.

12. Sep 16, 2015

### Eli137

OK I think I finally get it. Thank you.

13. Sep 17, 2015

### stevendaryl

Staff Emeritus
The big difference between QCD and QED is that the "force carrier" in QCD, the gluon, can itself have a color charge. In contrast, in QED, the force carrier, the photon, does not have an electric charge. So an electromagnetic interaction doesn't change the charge of a particle, but a QCD interaction can change the color of a particle.