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It has been 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 which are then injected into the ring. As all ring colliders, the LEP's ring consists of many magnets which force the charged particle on a circular trajectoryIn ordinary language, a trajectory is the path followed by a body moving through space, for instance, the path taken by a falling body or the orbit of a planet. One may either consider just the geometry of the path (i. the set of positions), or also the p (so that they stay inside the ring), rf accelerator s which accelerate the particles with radioFor other uses see: radio (disambiguation Radio is a technology that allows the transmission of signals by modulation of electromagnetic waves with frequencies below those of light. Radio waves Radio waves are a form of electromagnetic radiation, and are frequencyFrequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the l (rf) waves and quadrupole s that focusThe word focus (pl. foci has several meanings: focus (geometry) focus (optics) See also: autofocus, focal length focus (linguistics) Focus (band) focus (computing) Focus software Ford Focus (automobile) See also focal point (another disambiguation page). 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 energyKinetic energy (also called vis viva or living force is energy possessed by a body by virtue of its motion. The kinetic energy of a body is equal to the amount of work needed to establish its velocity and rotation, starting from rest. Equations Definition and so become more massive because massMass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. It is a central concept of classical mechanics and related subjects. Strictly speaking, there are two different quantities called mass Inertial mass and energyThis article is about the scientific concept. Energy use by humans is discussed in other articles''. Energy generally and qualitatively speaking, is the property (or the quantity of the property) of doing things or supplying power. The expressions 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 nearly immediatly decays into other elementary particles, which are then detected by huge particle detectors.
These detectors, built around the four collision points within underground halls, have each the size of a small house and are capable of registering the particles with their energy, momentum and charge, allowing physicists to infer the particle reaction that has happened and the elementary particles involved. By doing 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 to gat 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)-- and so confirm it and put it on a solid fundament of empirical data.
Near the end of the scheduled run time, data showed very tentative and 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 .