Introduction

The Standard Model of electroweak and strong interactions is the basis for all highenergy particle theory. It has achieved such wide acceptance that the primary test for the validity of a model in high-energy physics is that it yields this theory at sufficiently low energies. The Standard Model is an extension of the Weinberg—Salam model of the electroweak interactions to include quantum chromodynamics (QCD), the quark-gluon theory of the strong interactions.

The Weinberg—Salam model is a gauge theory, built around the group SU(2) × U(1). As a gauge theory, it has massless vector bosons for each symmetry generator. However, massless bosons correspond to long-range forces and would therefore have been detected long ago. Hence the theory includes a means for giving the vector bosons masses; it proposes the existence of some scalar fields (Higgs fields) and a gauge-invariant potential for them whose classical minima are only invariant under a U(1) subgroup of SU(2) × U(1). When the symmetry breaks, the vector field associated with the unbroken symmetry remains massless and is identified with the photon, but those associated with broken symmetries become massive, and we are reconciled with experimental evidence regarding massless vector bosons.