Quote by jmz34
Quick question, if there are n Feynman diagrams possible at first order say, is the rate proportional to n^2? I would have thought so since the matrix element is given by the expression M=M1+M2+M3+.... where 1,2 and 3 are the consecutive orders. I've been looking at a few examples that say the rate is proportional to n which is why I'm confused.

I wouldn't say that the question is one that can be answered in general, since to get the rate, we have to compute [tex]M^2[/tex] and integrate over phase space. Generally some diagrams will contribute more or less to the computation.
For instance [tex]e^+e^\rightarrow e^+e^[/tex] gets contributions from either an intermediate photon or [tex]Z^0[/tex] particle. When the energy of the initial particles is very small, the contribution from the intermediate [tex]Z^0[/tex] is extremely small.