This is an event display of a real-life K->2PI ("i.e., neutral Kaon decays to 2 charged pions.")
Top: View of the spectrometer, looking down from above.
Bottom: View of the spectrometer, looking from the side.
The charge pions from the Kaon decay live long enough to traverse the length of our experiment. The trajectories of each pion are measured using 4 drift chambers: 2 upstream and 2 downstream of the analysis magnet (the rectangle at 170 meters). The deflection of each pion in the magnet gives its momentum (P), and allows a reconstruction of the kaon mass. In this case it agrees quite well with the known value of 0.4977 GeV.
The fact that the supposed pion tracks don't penetrate the thick steel at at the downstream end of the experiment tells us that they are probably not muons, which are present in some of the more common background decays. The calorimeter is also used to reject backround, by comparing the energy (E) deposited in it to the momentum (P) of each track. A close match between E and P is a evidence that the particle is an electron, which are also present in many background decays.
This Kaon comes from the regenator beam (note the regenerator displayed at about 120 meters in the beam). This event is most likely a CP conserving K-short to 2PI decay.