The decay width is given by
and the total rate is the integral over this for the case 
.
For the case 
the integral has to be divided by the factor 2
for two identical particles in the final state.
We first consider the case where
and introduce the dimensionless variables
Then one obtains, after integrating over 
and 
at fixed 
[20],
This result allows one to evaluate, e.g.,
the distribution 
of produced 
pairs rather
easily. The kinematically allowed region is
with
The case 
is slightly more elaborate. We feel that it does not
make sense to display the term 
because it is of
considerable complexity in the general case when all the form factors
, 
and 
are 
and 
dependent. The expression
together with the Monte Carlo program to do the phase space integrals
is available on request from the authors.
