It is the detectors that enable us to measure the energy and trajectories of the particles that are created in the collisions. With this information the types of particles can be determined.

Three types of detector are commonly used:

• Closest to the centre are tracking detectors that measure the trajectories of charged particles. The tracking detectors are either wire chambers that consist of wires in a chamber filled with gas, or semiconductors consisting of a semiconducting material, e.g. silicon. Semiconductors have great precision but are expensive to manufacture and they are sensitive to radiation. Particles can penetrate the tracking detectors almost without being affected.
• In the next layer are the calorimeters that measure the energy of the particles. When a particle hits a calorimeter a shower of secondary particles is created. The size of the shower determines the energy of the first particle. There are two kinds of calorimeters. The inner one is an electromagnetic calorimeter that measures the energy of electrons, positrons and photons. The outer one is a hadron calorimeter that measures the energy of hadrons.
• Furthest out are muon detectors that like the calorimeters create showers of secondary particles. The muon detectors detect muons and anti-muons since these are the only particles that penetrate the calorimeters.

DELPHI was one of the detectors in the LEP accelerator; it consisted of the different types of detectors mentioned above. DELPHI has been removed to give room for ATLAS, which is larger detector of the same kind that will operate in the new LHC accelerator.