Another complication at LHC ist the existence of so-called background processes, many of which occur much more frequently than the processes for which we are looking. For events with an electron-positron or muon-antimuon pair, the most prominent backgrounds are pairs of top-antitop quarks. They can decay according to t+t→ μ++ν+ μ-+ν+… or t+t→ e++ν+ e-+ν+…, where … stands for further particles in quark jets. In order to suppress this background, one exploits characteristic differences to events with Z0 bosons