I'm looking for events where a W and a Z boson is produced from collision of two protons, with backgrounds coming from top-antitop production, production of 2 Z bosons and Drell-Yann processes (production of 1 Z boson). Selections have been made to reduce these backgrounds (one of those selections check for b-tagged jets and neglects events where one is present to reduce ttbar background, I also ask for 3 leptons which pass my selections).
Plotted here is the number of jets for each of these components (unstacked):
WZ and DY seem obvious: If you look at leptonic decays you don't expect jets, except those from initial state QCD radiation. But why is there a peak at 1 jet for ZZ production?
Same for ttbar: Not counting the jets coming from the top quarks, we don't expect jets except for initial and final state QCD radiation. Why then does it peak around 1 jet again? Is this just because final state radiation is also possible here in contrast to the case with the other components?