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
THE PARTICLE
ADVENTURE
This is a very good web site and where I found all the icons that appear on this page.
