In the previous section we did the spin spums by hand in order to
transform them in traces and than used FeynCalc to do the
traces. But this can handled in a different way which much
simpler. specially for many diagrams. We use the example above of
Bhabha Scattering (
) in QED
The relevant matrix element can be written as
where
with
.
Now we can use FeynCalc to perform the fermion spin sums for you. You have to use the FeynCalc function FermionSpinSum. The mathematica program that evaluates the spin sums using this technique is Bhabha-SpinSums.nb.