Like many events, this ATLAS event is unclear. The two towers in the lego plot (top right) and the yellow splotches in the green ring (main image) both indicate energy deposited by particles into the electromagnetic calorimeter. Which particles? This could be a low-energy electron-positron event, such as the decay of a J/psi particle. Not all events are easily understood, which is one reason particle physics is interesting, challenging but fun.