A WW decay were one of the W's has decayed into an electron and an electron-neutrino, and the second has decayed into a muon and a muon-neutrino. The green track (the electron) has deposited much energy in the EM-calorimeter (the inner circle). The muon, the read track, has been registered in the muon detector further out. Note that the two tracks are not at 180 degree from each other. This is due to the momenta of the two neutrinos that haven't been registered in the detector.