Some examples of Feynman amplitudes

A full and detmled calculation of a reaction rate, starting from a Feynman amplitude, is given in Section 8.6. 

for pedagogical reasons, we give a few examples of partonic Feynman diagrams and their corresponding ampUtudes. As usual for any 2 2 partonic process 

The factor (—1) in (A2.4.5) does not come from the Feynman rules. It comes from comparing the order of the fermion operators underlying the two diagrams. Symbolically we have  

To rearrange the operators in Ai(u) into the order occurring in M. t) requires three interchanges (i.e. an odd number) hence the relative (—1). 

In the above the colour b is, of course, summed over. Rules for doing colour sums are given in Section A2.5. To calculate the unpolarized cross-section involves forming 

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