Arrays of Detectors

Four large composite detectors envelope the LEP beam pipe at points where the electron and positron bunches cross. The aim of each is to detect as many ofthe particles produced in electron-positron annihilations as possible. To do this requires apparatus that surrounds the annihilation point. In addition, the physicists need to know which kinds of particle emerge and with what energies.They therefore use a variety of detectors that can assist in identifying different particles as well as in measuring their energies. These detectors are wrapped in layers around the beam pipe to form a single assembly at each offour points where annihilations occur.

View of the L3 detector at L.E.P.. Here the magnet doors are open to allow
physicists to work inside the detector.

Each detector assembly forms a huge structure, typically 10 to 12 metres high, wide and long - the size of a reasonably large house - and weighing several thousand tonnes. The four assemblies form separate experiments, codenamed ALEPHDELPHI, L3 and OPAL. Each is run by a team of around 200-300 physicists from around the world, with components coming from many different countries. Not only are all CERN's member states represented in the experiments at LEP, but there are also contributors from other countries such as China, Israel, Japan, the Russian Federation and the USA.

Three-Layer Detection

The detector assemblies have a similar basic design, although the details vary so that they complement each other by having different strengths The first layers of detectors (tracking detectors), closest to the beam pipe, reveal thetracks of charged particles (neutral particles do not leave tracks). A large electromagnet provides a magnetic field to bend these tracks, so that a particle's momentum can later be calculated from the amount of bending.

The DELPHI detector reveals two sprays of particles generated
by a quark and an antiquark created with an electron and a
positron annihilated in L.E.P..

Outside the tracking detectors, the next layer traps and identifies all electrons, positrons and photons as they plough into a dense material such as lead. This material is interleaved with detectors to measure the energy that the particles lose as they come to a halt. The aim is to create an electromagnetic calorimeter that measures all the energy of the electrons, positrons and photons. This helps in identifying particles such as neutral pions, which leave no tracks in the inner detectors, but which decay to photons. A third layer incorporates the iron that forms the outer part of the electromagnet. It stops the strongly interacting particles, or hadrons - that is, particles (mesons and baryons) built from quarks and antiquarks - and measures their energy. This layer forms a hadron calorimeter, since it measures the total energy of the particles.

The barrel of the electromagnetic calorimeter in the OPAL detector, containing
hundreds of blocks of lead glass.

Two kinds of particle are likely to penetrate beyond the hadron calorimeter: muons and neutrinos. The outermost layer of detectors reveals the tracks of penetrating charged particles, mainly muons. Only the neutrinos escape the apparatus without direct detection. But the physicists can infer the irexistence. They know the total energy of the electron and positron that annihilated. So if they add up all the energy deposited by particles in the various pieces of the apparatus, they can calculate, using the principle of conserving energy and momentum, how much energy has gone missing in the form of neutrinos, and in which directions the neutrinos have gone.

Crucial Circuitry

Electronics and computing play crucial roles in experiments of this kind. All the detectors used produce electrical signals, which electronic circuits convert into a form that can be fed into computers and stored on magnetic oroptical media. Other sophisticated circuits are necessary to process signals and make fast 'decisions' as to whether the information from an annihilation is worth recording. Such circuits act as triggers, which set off the whole complex chain for recording data from the experiment. Last, but by no means least, computers are necessary to take the recorded information and reconstruct what happened immediately after an annihilation, as the newly made particles flew out through the apparatus. It is from these 'events' that the physicists can eventually build up a picture of the underlying physical processes.

The OPAL detector demonstrates well the different roles of various layers that make up the whole assembly
  1. Vertex detector, to reveal decays in beam pipe
  2. Tracking detector
  3. Magnet coil, to bend tracks for momentum measurement
  4. Time-of-flight detector, for velocity measurement
  5. Electromagnetic calorimeter, to detect eletrons and photons
  6. Hadron calorimeter, to detect protons, pion etc..
  7. Muon detectors
  8. Forward detectors, for particles close to beam pipe