A Detector in Action

This is what a typical collision might look like in a typical detector. An electron coming in from the right collides with an anti-electron, positron, coming in from the left. They annihilate and produce a Z particle according to Einstein's famous formula E=mc2. The Z particle exists for just a fleeting moment before decaying into other particles, which fly out into the detector where they signal their passage through the different detector components.

Each particle leaves a distinct trace. Charged particles leave tracks in the inner layers, for example, whereas neutral ones do not. Some particles deposit all their energy in the innermost layer of the calorimeter, coloured green. Others punch their way through to the outer layer. Muons, charged particles that are particularly penetrating, leave tracks in the inner layers, a little energy in both parts of the calorimeter, and then they leave the detector. Some particles, neutrinos, hardly interact at all and leave no trace in the detector. Physicists deduce that they are there by adding up the energy of all the particles they see in the detector and comparing it to the energy of the initial electron and positron. Any missing energy must have been carried away by neutrinos... or else by particles we don't yet know about.

When you get to the projects, you will be analysing particle collisions that happened inside a detector called DELPHI, so on the next page we'll take a closer look at how the DELPHI detector works.