Introduction to Gauge Theories

These lectures present an elementary introduction to quantum gauge fields. The first aim is to show how, in the tree approximation, gauge invariance follows from covariance and unitarity. This leads to the standard construction of the Lagrangian by means of covariant derivatives in a form that unifies the massive and the massless case. Having so identified the classical theory, the Faddeev-Popov quantization method is introduced and the BRS invariance of the resulting action is discussed.