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 huge detector assemblies record the tracks of particles created in this way, and provide the physicists with glimpses of the behaviour of matter at high energies.
LEP is a circular machine, as big as the Circle Line on London's Underground, and a direct descendant of the first accelerators that Lawrence and Livingston built at Berkeley almost 60 years ago. It has a ring of magnets to guide the bunches of particles on a circular path around a narrow pipe, so that they pass repeatedly through regions where they are given small accelerating boosts. But in LEP the two types of particle - four bunches of electrons and four bunches of positrons - travel in opposite directions around the ring. Once the particles have reached maximum energy, the paths of the particles are allowed to cross at four points so that some of the electrons and positrons can collide (although most of the particles from each pair of colliding bunches do not come close enough to annihilate). So, once it has accelerated the particles, LEP stores the bunches allowing them to speed round the machine for several hours, colliding every 22 millionths of a second.
LEP is the largest accelerator built so far. It lies in a tunnel, 3.8 metres wide, which forms a ring 27km in circumference. The tunnel extends out from CERN's main site at Meyrin in Switzerland, crossing the border as it loops under French countryside to the foothills of the Jura mountains and back. It contains 4600 magnets to guide the particle beams. There are also two sections where the particles are accelerated by radio waves set up in hollow copper structures, called cavities.
The particle beams travel around the magnet ring within a beam pipe - 27km of aluminium tube, which has to be held at a very high vacuum so that collisions with stray molecules do not knock the beam particles off course. The large ring is necessary to keep the particles on a gently curving path.
Electrons and positrons radiate energy as their paths bend, the amount of energy lost in this way increasing both with the energy of the particles themselves and with the curvature of the path. At LEP's high energies, the curvature must be as small as possible to minimise the radiation losses.
The electrons and positrons enter LEP after a journey through smaller accelerators, which raise their energy in stages. Two of these machines were built originally to accelerate protons, under the guidance of John Adams, a British engineer, who was twice director-general of CERN. The smaller of the two started up in 1959, and continues to run as the centre-piece in a network of machines that can supply beams of protons, antiprotons, sulphur or lead nuclei, as well as electrons and positrons. Following in Adams's footsteps, British engineers and physicists continue to figure prominently in the running of CERN's accelerators, as well as in successive new developments.