e+e- -> Z -> tau+ + tau- -> e+ + mu- (+ 4 neutrinos)
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In this event, the Z particle decays into a tau-antitau pair. Taus are unstable particles and decay inside the detector producing events like this one. In this case, the antitau has decayed into a positron, and the tau into a muon. Two neutrinos are also produced from each decay, but these are not detected.

Tau leptons do not always decay into only one visible particle. Sometimes they produce two or three. This means that they may look like quark-antiquark events. If you see an event which has something that looks like a jet on one side but a single particle on the other side, it has to be a tau-antitau event. If you see an event with two jets, each of which only has only two or three particles in it, you should consider whether it might be a tau-antitau event and not a quark-antiquark pair.

Tau-antitau events can also be confused with electron-positron events or muon-antimuon events. The way you can distiguish them is by carefully identifying each particle. If they both look like electrons, then you have an electron-positron pair. If they both look like muons, then you have a muon-antimuon pair. But if you have an electron and a muon together, for example, then you are probably looking at a tau-antitau event.

Particle Physics Education CD-ROM 1999 CERN