\midheading{$Zt{\bar t}$ production, processes 529-533}
\label{subsec:ztt}

These processes represent the production of a $Z$ boson in association
with a pair of top quarks. These processes are only available at LO.
For process {\tt 529}, neither the top quarks nor the $Z$-boson
decays.
In processes {\tt 530-533}, the top quarks are always
produced on-shell, which is a necessity for a gauge invariant result
from this limited set of diagrams.
Switching {\tt zerowidth} from {\tt .true.} to {\tt .false.} only affects
the $Z$ and the $W$ bosons from the top quark decay.
In process {\tt 530} the $Z$ boson decays into an electron pair, whilst
in {\tt 531} the decay is into a massless bottom quark pair.
In process {\tt 532--533} the $Z$ boson decays into an electron pair, whilst
one or other of the top quark or top anti-quark decays hadronically.

For processes {\tt 532} and {\tt 533} the default behaviour is that the hadronic decay products
are clustered into jets using the supplied jet
algorithm parameters, but no cut is applied on the number of jets.
This behaviour can be altered by changing the value of the
variable {\tt notag} in the file {\tt src/User/setnotag.f}.

When {\tt removebr} is true in process {\tt 530}, the top quarks and the $Z$ boson do not decay.