After an accelerator has pumped enough energy into its particles, they collide either with a target or each other. Each of these collisions is called an event. The physicist's goal is to isolate each event, collect data from it, and check whether the particle processes of that event agree with the theory they are testing.

Each event is very complicated since lots of particles are produced. Most of these particles have lifetimes so short that they go an extremely short distance before decaying into other particles, and therefore leave no detectable tracks.

How can a physicist determine what happened if she can never record the presence of several key particles?