This is an event display of a real-life K->2PI0 ("i.e., neutral Kaon decays to 2 neutral pions.")
Top: View of the CsI calorimeter, looking along the beams.
Bottom: View of the experiment, looking down from above.
The neutral pions each decay immediately to two photons, which are then detected in the CsI calorimeter as clusters of energy. The reconstruction of the event is performed by assuming that the photons came from neutral pions, and finding the combination most consistent with the pions decaying at the same point in space. The kaon mass, calculated from angles and energies of the photons, agrees very well with the known value of 0.4977 GeV.
This Kaon comes from the vacuum beam (note the regenerator displayed at about 120 meters in the other beam). This event is almost certainly a CP violating K-long to 2PI decay.