Adding Color

Abstract

The quark model solved many of the problems of elementary particle physics. The hundreds of seemingly unrelated “elementary” particles and resonances were now understood. But searches were made for isolated quarks and none were found. Why? Perhaps they were forever confined to the interior of the hadrons as Gell-Mann had suggested. Many physicists were skeptical of the whole idea, yet the predictions of the theory were amazingly accurate. And they couldn’t just be brushed aside. Still, there were problems—a particularly glaring one related to the spin of the particle. I talked earlier about spin, pointing out that the electron has spin ½. Other particles such as the pion, on the other hand, have spin 0. We can, in fact, group particles into two classes according to their spin. The first class, those with half-integral spin (i.e., ½, 3/2,...), are called fermions; the second class, those with integral spin (0, 1, 2,...) are called bosons. Electrons, obviously, are fermions; pions are bosons.