Quantum Chromodynamics (QCD)

Quantum chromodynamics (QCD) is the theory of the strong interaction between the quarks that constitute the strongly interacting particles, the hadrons, consisting of baryons and mesons. (► Particle Physics. Quarks, see ► Color Charge Degree of Freedom in Particle Physics; Mixing and Oscillations of Particles; Particle Physics; Parton Model; QFT). It is modeled on the extremely successful theory of ► electrons and photons, quantum electrodynamics (► QED). However, unlike the latter, which is tested now to 10th order in the strength of the electric charge of the electron, it is not easy to compare QCD with experiment. This is not only because the coupling is strong, not weak as in electrodynamics, but also because of the related fact that the fundamental components of the theory, the quarks and the force-carrying gluons, have never been directly seen, and are generally believed to be unattainable because of the phenomenon of confinement.

Yukawa [1] was the first to start to understand the strong force in terms of his posited “;mesotron,” what we now call the pion. He believed that the strong nuclear force between protons and neutrons in the nucleus could be understood in terms of the exchange of a mesotron between these particles, the short range of the nuclear force reflecting the fact that the mass of the mesotron was around 100MeVc ⁻². However, by the end of the 1950s dozens of strongly interacting particles, most rapidly decaying, had been discovered in cosmic rays and accelerators, and these could not all be fundamental. Physicists searched for various schemes to unite the zoo of particles, and Gell-Mann [2] and Zweig [3] independently came up with the quark model, which was first not taken very seriously except as a way to describe the group theory that organized the hadrons. This group was called by Gell-Mann the eightfold way, but in fact it was simply SU(3), the group of three by three unitary matrices with determinant one. The fundamental representation of the group was realized by three quarks, what we now call up, down, and strange. (Now we know there are six “flavors” of quarks, up, down; charm, strange; and top, bottom; grouped in three families or generations of two each.) The quarks had fractional charge; up had charge +2/3, down had charge −1/3 in units of the electron charge, and each carried► spin ħ/2.