In this event, two tau particles are produced from the electron - positron annihilation. These are leptons, like the electron and the muon, though the tau is about 3500 times heavier than the electron. They are unstable particles and decay inside the detector producing events like the one shown. In this case, the anti-tau has decayed into a positron, and the tau into a muon. 2 neutrinos are also produced from each tau decay, but these interact very weakly, and are therefore not shown on the diagram. They pass straight through the detector.

Notes for experts: The positron momentum is measured to be 37.1 GeV/c in the tracking chambers and has an associated energy in the HPC of 30.0 GeV. It has no (or little) energy deposited in the hadron calorimeter and is thus a clearly identified positron. The muon has a 25.4 GeV/c momentum. It is not observed in the HPC and is minimum ionizing in the hadron calorimeter. It has associated hits in both planes of the barrel muon chambers. It is a clear muon. There is about 30 GeV energy missing in the event, which is carried away by the neutrinos.