Large Electron–Positron Collider

"LEP" redirects here. For other uses, see LEP (disambiguation).

The former LEP tunnel at CERN being filled with magnets for the LHC

The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed.

It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland. LEP was a circular collider with a circumference of 27 kilometres built in a tunnel straddling the border of Switzerland and France. It was used from 1989 until 2000. To date, LEP is the most powerful accelerator of leptons ever built.

History

When the LEP collider started operation in August 1989 it accelerated the electrons and positrons to a total energy of 45 GeV each to enable production of the Z boson, which has a mass of 91 GeV. The accelerator was upgraded later to enable production of a pair of W bosons, each having a mass of 80 GeV. LEP collider energy eventually topped at 209 GeV at the end in 2000. At a Lorentz factor ( = particle energy/rest mass = [104.5 GeV/0.511 MeV]) of over 200,000, LEP still holds the particle accelerator speed record, extremely close to the limiting speed of light. 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).

Operation

An old RF cavity from LEP, now on display at the Microcosm exhibit at CERN

The Super Proton Synchrotron (an older ring collider) was 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). The function of the accelerators is to increase the particles' energies so that heavy particles can be created when the particles collide. 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.

Detectors

The Large Electron–Positron Collider had four detectors, built around the four collision points within underground halls. Each was the size of a small house and was capable of registering the particles by their energy, momentum and charge, thus allowing physicists to infer the particle reaction that had 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 were called Aleph, Delphi, Opal, and L3. They were built differently to allow for complementary experiments.

ALEPH

ALEPH stands for Apparatus for LEP PHysics at CERN. The detector determined the mass of the W-boson and Z-boson to within one part in a thousand. The number of families of particles with light neutrinos was determined to be 2.982±0.013, which is consistent with the standard model value of 3. The running of the quantum chromodynamics (QCD) coupling constant was measured at various energies and found to run in accordance with perturbative calculations in QCD.

DELPHI

Main article: DELPHI experiment

DELPHI stands for DEtector with Lepton, Photon and Hadron Identification.

OPAL

OPAL stands for Omni-Purpose Apparatus for LEP. The detector was dismantled in 2000 to make way for LHC equipment. The lead glass blocks from the OPAL barrel electromagnetic calorimeter are currently being re-used by the NA62 experiment at CERN. The name was a pun since some of the founding members of the scientific collaboration which first proposed the design had previously worked on the JADE detector at DESY in Hamburg.

L3

L3 was another LEP experiment. Its enormous octagonal magnet return yoke remained in place in the cavern and became part of the ALICE detector for the LHC.

Results

The results of the LEP experiments 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.

An unfinished discovery of the Higgs boson

Near the end of the scheduled run time, data suggested very tentative but inconclusive hints that the Higgs particle of a mass around 115 GeV 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. The strength of the signal remained at 1.7 standard deviations which translates to the 91% confidence level, much less than the confidence expected by particle physicists to claim a discovery. Subsequent experiments until 2010 at the Tevatron had not been sensitive enough to confirm or refute these hints. In December 2011, the ATLAS and CMS experiments at LHC, the successor of LEP, presented stronger (than LEP) hints of a 125 GeV Higgs. These announcements were followed up by both ATLAS and CMS in July 2012, when a particle discovery was announced at around 125 to 126 GeV. During the July 2012 announcements, the 115 GeV region was strongly excluded, casting greater doubt on the earlier LEP observation.

See also

• Electron-positron annihilation
• Large Hadron Collider

External links

• LEP Working Groups
• The LEP Collider from Design to Approval and Commissioning excerpts from the John Adams memorial lecture delivered at CERN on 26 November 1990
• 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.