NONPERTURBATIVE STUDY OF THE FERMION PROPAGATOR IN QUENCHED QED IN COVARIANT GAUGES USING A RENORMALIZABLE TRUNCATION OF THE SCHWINGER-DYSON EQUATION

The Schwinger-Dyson equation for the fermion propagator in quenched fo ur-dimensional QED is solved using a nonperturbative ansatz for the fe rmion-photon vertex that satisfies the Ward-Takahashi identity, ensure s the multiplicative renormalizability of the fermion equation, and re produces low-order perturbation theory in the appropriate limit. The f ermion propagator then possesses a chiral-symmetry-breaking phase only when the coupling a is larger than a critical value alpha(c) congruen t-to 0.92. This critical coupling is almost exactly gauge independent as is the dynamically generated mass, in complete contrast to the popu lar rainbow approximation.