\midheading{$t \bar{t}$ production mediated by $Z/\gamma^*$-boson exchange, process 36}

These processes represent the production of a virtual $Z$ boson or photon
which subsequently decays into $t \bar{t}$.
The leptonic decays of the top quarks are included.
Switching {\tt zerowidth} from {\tt .true.} to {\tt .false.} only affects
the $W$ bosons from the top quark decay.
Note that {\tt m34min > 0} is obligatory due to the inclusion of the
virtual photon diagrams. The calculation may be only be performed at LO.
This is a small contribution, but in $p\bar{p}$-collisions gives a forward backward asymmetry.