Section 10.6 Chromodynamics: The Theory of Colored Quarks
Let's now investigate the strong interaction between quarks. The strong interaction is actually a manifestation of the color property of quarks. Just as with electric charges, color charges repel if alike and attract if different. Now a quark, like any particle of quantum field theory, continually emits and absorbs messenger particles and is thus surrounded by a small cloud of virtual quarks and gluons. Nearby quarks then detect the presence of the first quark by interacting with its cloud of colored gluons.
It is important to understand what happens to a quark when emitting or absorbing gluons: as a quark emits a gluon, its color changes. This color change can be seen in Figure 10.4. Note that a gluon carries both a color and an anticolor. In the example, the red up quark on the left emits a red-antiblue gluon and changes into a blue up quark. You can think of this by saying that the red color leaves the quark and goes into the gluon. The antiblue color of the gluon is also withdrawn from the quark, leaving the quark blue. The gluon may then carry color to another quark, or it may return to the original quark to be re-absorbed.
An important aspect of the color force between quarks is how its interaction strength varies with distance. We can best understand this concept through an analogy with electric forces. As you recall from our study of Gauss' Law, the electric flux through a surface depends only on the net charge inside the enclosed volume. Similarly, the color field depends only on the net color inside a given volume. We must thus consider the effect of the gluon messenger particles, since they can carry the color charge away from a quark.
The color carried by a gluon leads to the effect called camouflage. Consider a red quark. Suppose while you are trying to test the color inside a certain region surrounding the quark, it emits a red-antigreen gluon, leaving behind a green quark. The gluon can't go far without violating the uncertainty principle and unless it finds another quark, it soon returns and is re-absorbed. But during the time it's away, the net color in a large region enclosing the fleeing gluon is still red, while in a smaller region missing the gluon, the net color of the quark looks green. The combined effect of gluons continually leaving and returning is that the closer you get to the red, the less red it appears. Thus the effective color charge decreases as you move closer to it.
Detailed calculations show that this camouflage effect makes the color charge continually decrease at smaller and smaller separations. In fact, it turns out that on distance scales smaller than a proton radius, the effective quark color charge is so diminished that the three different colored quarks inside a proton hardly affect each other and move almost independently. The gluon force between quarks thus acts like a rubber band tied between two balls. The balls move freely until the rubber band gets stretched. Then the more they try to move apart, the harder the rubber band force pulls them back together. Continuing the rubber-band analogy, you can see that, as you try to separate quarks, the effective color charge increases rapidly and the inter quark forces become huge. In fact, the energy needed to pull apart quarks is so great that a quark-antiquark pair is created instead, as shown in Figure 10.5. Instead of a free quark, we just create a new meson. This is why we've never observed free quarks; all quarks are permanently confined inside hadrons.
