7 - The advent of QCD

Summary

The underlying idea of current algebra, light-cone current algebra included, was to exploit the broken symmetry of strong interactions. The idea was pursued through abstracting physical predictions, as the consequences of the symmetry and in the form of certain algebraic relations obeyed by weak and electromagnetic currents of hadrons to all orders in strong interactions, from some underlying mathematical field theoretical models of hadrons and their interactions, more specifically from models of quark fields universally coupled to a neutral gluon field.

When current algebra was initially proposed, the symmetry was defined with respect to static properties of hadrons, such as mass, spin, charge, isospin, and strangeness. However, over the years, especially after the confirmation of scaling, in the changed theoretical and experimental context, the notion of symmetry underlying the current algebra project had undergone a transmutation. A new component of symmetry, that is, a dynamic symmetry, was brought into the scheme. The result was not only some adaptations in algebraic relations and corresponding physical predictions, but also a radical change in the nature of the underlying field theoretical model. More precisely, the result was the emergence of a gauge invariant quark–gluon model for strong interactions, or the genesis of quantum chromodynamics (QCD).