Instead of flying out and being observed in the calorimeter, sometimes the photon produces an additional particle-antiparticle pair. Such events contain two particle-antiparticle pairs. For example:
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These events go by the jargon name "four fermion" events. Here is an example event that contains electron-positron and muon-antimuon pairs:
Back to general section on "How to Identify Some Slightly More Complicated Types of Event".