NONPERTURBATIVE 3-POINT VERTEX IN MASSLESS QUENCHED QED AND PERTURBATION-THEORY CONSTRAINTS

Dong, Munczek, and Roberts have shown how the full 3-point vertex that appears in the Schwinger-Dyson equation for the fermion propagator ca n be expressed in terms of a constrained function W-1 in massless quen ched QED. However, this analysis involved two key assumptions: that th e fermion anomalous dimension vanishes in the Landau gauge and that th e transverse vertex has a simplified dependence on momenta. Here we re move these assumptions and find the general form for a new constrained function U-1 that ensures the multiplicative renormalizability of the fermion propagator nonperturbatively. We then study the restriction i mposed on U-1 by recent perturbative calculations of the vertex and co mpute its leading logarithmic expansion. Since U-1 should reduce to th is expansion in the weak coupling regime, this should serve as a guide to its nonperturbative construction. We comment on the perturbative r ealization of the constraints on U-1.