DYSON SUMMATION WITHOUT VIOLATING WARD IDENTITIES AND THE GOLDSTONE-BOSON EQUIVALENCE THEOREM

In contrast with the conventional treatment of gauge theories, in the background-field method the Ward identities for connected Green functi ons are not violated by Dyson summation of self-energies in finite ord ers of perturbation theory. Thus, Dyson summation does not spoil gauge cancellations at high energies which are ruled by the Goldstone-boson equivalence theorem. Moreover, in the background-field method the pre cise formulation of the equivalence theorem in higher orders (includin g questions of renormalization) is simplified rendering actual calcula tions easier. Finally, the equivalence theorem is also formulated for the standard model with a nonlinearly realized scalar sector and for t he gauged nonlinear a model.