We start with the processes (2.1) and (2.2),
The matrix element is
where
with
The form factors 
are the ones defined
in appendix C.
is the electromagnetic form factor
of the 
. Finally the quantity 
stands
for
and the four-momenta are
such that 
.
In order to obtain the matrix element for (2.3) and (2.4),
one identifies 
and 
in (2.6) and subtracts the
contribution obtained from interchanging 
:
