CERN is the "Conseil Européen pour la Recherche Nucléaire" or in English the European Organization for Nuclear Research. CERN has made many significant discoveries over the years many of which using the Large Electron-Positron Collider (LEP), the predecesor of the Large Hadron Collider.
LEP collides together bunches of electrons with bunches of positrons, as they travel in opposite directions round a ring 27km in circumference, at velocities close to the speed of light. When the bunches of particles meet, some electrons and positrons annihilate, creating, for a fraction of a second, bursts of high energy which echo the state of the early Universe, but are quite harmless. Almost instantaneously the energy rematerialises as streams of subatomic particles.
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.
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 ALEPH, DELPHI, 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 all the scientists are from European countries, there are also contributors from other countries such as China, Israel, Japan, the Russian Federation and the USA.