Beyond the Classical Approximation
We have seen in Chap. 4 that physical transition amplitudes correspond to amputated connected diagrams with a given number of external lines. In general, these diagrams contain loops as discussed in Sect. 4.5. Following the Feynman rules given in Chap. 4, a generic connected diagram can be built as a tree diagram whose vertices are either point-like, or sub-diagrams with loops. We shall call these vertices effective vertices: an effective vertex is a sub-diagram that cannot be broken into two parts cutting a single internal line.