The colour field

The gauge symmetry explanation of the strong force ended in frustration: so near, and yet so far! If only those pions hadn't taken it into their little heads to be massive spinless particles, instead of being vector bosons as we would have liked. But maybe not all is lost; the isospin symmetry is unmistakable, because the similarities between the neutron and the proton are readily observable, and the pion triplet does smell strongly of SU(2). Maybe we can keep most of this, and just allow the possibility that the pion is a scalar particle because it is composite: if it consists of a spin up–spin down pair, it can have total spin zero. Also, if the pions are composites, it is no longer unexpected that they decay, even though the phenomenal difference between the π+− and π0 lifetimes needs an explanation. Maybe there is more structure in the strong force than meets the eye.

If the pions are composites, then we must assume that the nucleons are composites, too. Collectively, the pions, nucleons and their relatives – which we will meet anon – are called hadrons. What are the quanta of which the hadrons are made? In other words, what is the fermion multiplet from which we must build them? Consider an analogy with the local U(1) symmetry that produced the photon, electromagnetism and electrically bound composites such as atoms.