Continuing with our discussion of the integration of quantum mechanics with the forces of nature, today's topic is Quantum Chromodynamics (QCD), the quantum theory of the strong nuclear force. As discussed previously, the matter particles of the strong force are quarks, whereas the force-carrying particles are gluons (see the March and April 2005 archives, in the lower right-hand portion of the page). On the Particle Adventure website, the section on "What Holds it Together?" nicely describes the operation of the strong force, as well as the other forces.
Hadrons are combinations of quarks (and/or antiquarks; see here for description of antiparticles, which are symbolized by a horizontal bar over the letter symbol for a given particle).
Two types of hadrons are baryons, which consist of three quarks, and mesons, which consist of a quark-antiquark pair. One property of quarks, discussed earlier, is "flavor": up, down, strange, charmed, top, and bottom. Probably the best known baryons are protons (two up quarks and a down quark) and neutrons (two downs and an up).
An additional property of quarks is "color" (which really has nothing to do with pigmentation). That is where the "chromo" in QCD comes from. Combinations of quarks must "add" up to "white" or "neutral." Quoting from a Caltech document:
...the strong force comes in three forms of charge, called colors: blue, green, and red. These aren’t real colors visible to the eye, of course, but they do exhibit a similar bit of behavior—one blue, one green, and one red quark add up to be colorless, just as equal parts of blue, green, and red light add up to white light. All observable particles—your protons, neutrons, pions, kaons, and what have you—are color-neutral. And just as all particles, including quarks, have antiparticles, colors have anticolors: antiblue (yellow), antigreen (magenta), and antired (cyan). A bound pair of a color and its anticolor is also color-neutral.
These combinations are illustrated in a color wheel on the Caltech document.
A baryon (with three quarks) can thus consist of a blue, a green, and a red quark, whereas an antibaryon would consist of an antiblue, antigreen, and antired quark. A meson (quark-antiquark pair) could be blue-antiblue, green-antigreen, etc. Pions and kaons, which were noted above, are both types of mesons.
Flavor and color can occur in all possible combinations for a quark (e.g., red-up, green-up, blue-down). According to a New Mexico Tech document, "Counting all color and flavor combinations, there are 6 X 3 = 18 known varieties of quarks."
Andrew Watson's book The Quantum Quark provides a thorough overview of QCD. According to this book:
The simple blunt truth is that without color there would be no QCD since, according to QCD, color is the particle attribute through which those particles interact. That means experimenters need to come up with some pretty rock-solid evidence for color. And not just that: they must be able to show that there are three colors, no more and no less.
Experimenters have risen to the challenge. They have produced support for the existence of color, or at least something that behaves in just the way folk expect color to behave. They have also found evidence that the number of colors really is three (p. 183, my emphasis added).
As the Wikipedia document on QCD notes, two of its key aspects are asymptotic freedom (for which David Gross, Frank Wilczek, and David Politzer received the Nobel Prize) and confinement.
My perusal of some dictionary definitions of the word "asymptote" on the web suggests that it refers to approaching a limit, tending to zero, or getting close to something. In this context, the description of asymptotic freedom in the aforementioned Nobel Prize announcement is clear:
...the closer the quarks are to each other, the weaker is the 'colour charge'. When the quarks are really close to each other, the force is so weak that they behave almost as free particles. This phenomenon is called ”asymptotic freedom”. The converse is true when the quarks move apart: the force becomes stronger when the distance increases. This property may be compared to a rubber band. The more the band is stretched, the stronger the force.
Confinement means that single quarks cannot be observed alone and must join with other quarks or an antiquark. According to Physics Today:
Try to pry loose one of the three valence quarks in a proton. Before going much farther than the radius of the proton (about 1 fm or 10-13 cm), you've done enough work to create a new quark-antiquark pair. Pairs promptly appear, choose new partners, and you find a meson in one hand and a proton or neutron in the other. No isolated quarks! (The 10-13 should be read as 10 the minus 13th power.)
Quantum Chromodynamics, the quantum theory of the strong nuclear force, thus appears to be a thriving, empirically supported area of research. Linking quantum mechanics to the remaining force -- gravity -- is still under development, and will be examined in my next posting.
