What is QCD???

This interactive animation uses a Java applet called a Physlet to simulate the interaction of quarks.  Note that the quarks experience a large attractive force when they are far apart (confinement) and are essentially free (asymptotic freedom) when they are close together.

QCD stands for Quantum Chromodynamics.  This is the area of particle theory in which I have worked with the late Kurt Haller at the University of Connecticut.  It is also the area of particle physics in which David J. Gross (Kavli Institute, University of California, Santa Barbara), H. David Politzer (Caltech), and Frank Wilczek (MIT) received the 2004 Nobel Prize in Physics.

Quantum Chromodynamics is the theory of how quarks and gluons interact with themselves and each other. The word quantum stands for the fact that interactions (forces between particles) on this level can be represented as particles that occur only in chunks called quanta. As a consequence, energy can only change by these bits. Gluons are the particles that mediate the force (the strong interaction) in QCD.  In the process of constructing the theory, quarks and gluons are quantized allowing the `creation' of individual quarks and gluons.

Both quarks and gluons carry a type of charge called 'color.'  Like electric charge, color charge is always conserved.  But unlike the electric charge, the color charge (the chromo in chromodynamics) comes in six varieties, three colors and three anti-colors. The colors are usually called red, green, and blue. The idea is that we know that protons and neutrons (as well as many other particles called hadrons) are made up of quarks.  Yet we never see color charge even if we try to break up protons and neutrons into their constituent parts (colored quarks). So the objects that we observe, and therefore construct, must be colorless or color neutral; which is why we cannot see individual quarks.  When each quark in a hadron has a different color, red+green+blue=white, the result is a color neutral object.  This also allows the quark picture to describe another class of particles (mesons) which have a quark and an anti-quark (color+anti-color=white).  Gluons carry color/anti-color pairs that do not have to be the same color. There are 8 gluons as they each have one of the eight possible color/anti-color combinations.

Quarks and gluons, as stated above, are colored particles. Colored particles exchange gluons. Quarks constantly change their color charge as they exchange gluons (interact) with other quarks.

You can think of the gluons linking quarks together in a hadron (or meson) as springs that act to maintain the color neutrality of the hadron (or meson). The harder you pull on a spring, the harder it pulls back. So if one of the quarks in a hadron moves from its neighbors, it is pulled back into place by the color force. In exchanging gluons, these colored particles are often glued together, which is called confinement. This is why individual colored particles cannot be found and why quarks combine into baryons (three quark objects) and mesons (quark-antiquark objects).

However, if one of the quarks in a hadron is pulled away from its neighbors hard enough, we can break the spring, creating two objects, each of which is still colorless.

This is similar to breaking a magnet in half. Originally it has a north and south, after breaking it each piece has a north and south. The spring breaks into two new quarks (conserving color and color neutrality), just as breaking the magnet creates two new poles.

So how do protons stick together in a nucleus? The colored quarks of one proton can glue themselves to the colored quarks of another proton, even though the protons themselves are color neutral. This is called the residual strong interaction. It is strong enough to overcome the electromagnetic repulsion between protons.

If you are interested in the rest of particle physics I would suggest browsing