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 529, neither the top quarks nor the Z-boson decays. In processes 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 zerowidth from .true. to .false. only affects the Z and the W bosons from the top quark decay. In process 530 the Z boson decays into an electron pair, whilst in 531 the decay is into a massless bottom quark pair. In process 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 532 and 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 notag in the file src/User/setnotag.f.
When removebr is true in process 530, the top quarks and the Z boson do not decay.
nplotter_ttZ.f is the default plotting routine.