Large Electron Positron Collider

Formerly the world's largest particle accelerator, in operation 1989–2000 at the CERN laboratories near Geneva in Switzerland. It occupied a tunnel 3.8 m/12.5 ft wide and 27 km/16.7 mi long, which is buried 180 m/590 ft underground and forms a ring consisting of eight curved and eight straight sections. In June 1996, LEP resumed operation after a £210 million upgrade. The upgraded machine, known as LEP2, generated collision energies of 161 gigaelectron volts.

Electrons and positrons entered the ring after passing through the Super Proton Synchrotron accelerator. They travelled in opposite directions around the ring, guided by 3,328 bending magnets and kept within tight beams by 1,272 focusing magnets. As they passed through the straight sections, the particles were accelerated by a pulse of radio energy. Once sufficient energy had been accumulated, the beams were allowed to collide. Four giant detectors were used to study the resulting shower of particles. In 1989 the LEP was used to measure the masses and lifetimes of the W and Z bosons, carriers of the weak nuclear force. The LEP has now been removed from its tunnel to make way for another particle accelerator, the Large Hadron Collider (LHC).