The Large Electron-Positron Collider (usually called LEP for short. It was dubbed Lots of Extra Problems during the initialisation and calibration phases) is one of the largest particle accelerators finished so far.

It was built at CERN, a multi-national center for research in nuclear and particle physics, located near Geneva, Switzerland.

LEP is a giant evacuated ring with a circumference of 27 kilometers built in an underground tunnel under the border of Switzerland and France. It was used from 1989 until 2000.

The Super Proton Synchrotron (an older ring collider) is used to accelerate electrons and positrons to nearly the speed of light. These are then injected into the ring. As in all ring colliders, the LEP's ring consists of many magnets which force the charged particles into a circular trajectory (so that they stay inside the ring), RF accelerators which accelerate the particles with radio frequency (RF) waves and quadrupoles that focus the particle beam (i.e. keep the particles together). (Note that 'accelerating' here does not really mean that the particles get faster, as they already are very close to the speed of light in the beginning. But they get kinetic energy and so become more massive because mass and energy is the same according to the theory of special relativity.)

When the LEP collider started operation in 1989 it accelerated the electrons and positrons to a total energy of 45 GeV. That means that the mass of each electron, in rest 511 keV, is boosted by a factor of 90,000. Further improvement of the devices increased this energy even further, topping at 106 GeV at the end in 2003.

When the particles are accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch is made to collide with each other at one of the collision points of the detector. When an electron and a positron collide, they annihilate to a virtual particle, either a photon or a Z boson. The virtual particle almost immediately decays into other elementary particles, which are then detected by huge particle detectors.

These detectors, built around the four collision points within underground halls, are each the size of a small house and are capable of registering the particles by their energy, momentum and charge, thus allowing physicists to infer the particle reaction that has happened and the elementary particles involved. By performing statistical analysis of this data, knowledge about elementary particle physics is gained.

The four detectors of LEP are called ALEPH, DELPHI, OPAL, and L3. They all are built differently to allow for complementary experiments.

The results of the LEP exeriments allowed precise values of many quantities of the Standard Model -- most importantly the mass of the Z boson and the W boson (which were discovered in 1983 at an earlier CERN collider)to be obtained -- and so confirm the Model and put it on a solid basis of empirical data.

Near the end of the scheduled run time, data suggested very tentative but inconclusive hints that the Higgs particle might have been observed, a sort of Holy Grail of current high-energy physics. The run-time was extended for a few months, to no avail.

At the end of 2000, LEP was shut down and then dismantled in order to make room in the tunnel for the construction of the Large Hadron Collider (LHC).

A short but good (though slightly outdated) overview (with nice photographs) about LEP and related subjects can be found in this online booklet of the British Particle Physics and Astronomy Research Council .