Structure of the Vertex Function

Stanley Deser, Walter Gilbert, E. C. G. Sudarshan
1959 Physical Review  
An integral representation as a function of invariants is found for the Fourier transform of the matrix element between the vacuum and a one-particle state of the retarded commutator of two currents. A special case is a spectral representation for the vertex as a function of momentum transfer. The threshold in this representation is lower than that found in the usual perturbation theory.
doi:10.1103/physrev.115.731 fatcat:3oky5ntd4vbphle2us3u5ixwom