 | |
| General properties | |
|---|---|
| Accelerator type | Synchrotron |
| Beam type | Electrons,positrons |
| Target type | Collider |
| Beam properties | |
| Maximum energy | 209 GeV |
| Maximum current | 6.2 mA |
| Maximumluminosity | 1×1032/(cm2⋅s)[1] |
| Physical properties | |
| Circumference | 26659 m |
| Location | Geneva, Switzerland |
| Coordinates | 46°14′06″N06°02′42″E / 46.23500°N 6.04500°E /46.23500; 6.04500 |
| Institution | CERN |
| Dates of operation | 1989–2000 |
| Succeeded by | Large Hadron Collider |
TheLarge Electron–Positron Collider (LEP) was one of the largestparticle accelerators ever constructed. It was built atCERN, a multi-national centre for research in nuclear and particle physics nearGeneva, Switzerland.
LEP collidedelectrons withpositrons at energies that reached 209 GeV. It was a circular collider with acircumference of 27 kilometres built in a tunnel roughly 100 m (300 ft) underground and passing throughSwitzerland andFrance. LEP was used from 1989 until 2000. Around 2001 it was dismantled to make way for theLarge Hadron Collider, which re-used the LEP tunnel. To date, LEP is the most powerful accelerator ofleptons ever built.
LEP was a circular lepton collider – the most powerful such ever built. For context, modern colliders can be generally categorized based on their shape (circular or linear) and on what types of particles they accelerate and collide (leptons or hadrons).Leptons are point particles and are relatively light. Because they are point particles, their collisions are clean and amenable to precise measurements; however, because they are light, the collisions cannot reach the same energy that can be achieved with heavier particles.Hadrons are composite particles (composed of quarks) and are relatively heavy; protons, for example, have a mass 2000 times greater than electrons. Because of their higher mass, they can be accelerated to much higher energies, which is the key to directly observing new particles or interactions that are not predicted by currently accepted theories. However, hadron collisions are very messy (there are often many unrelated tracks, for example, and it is not straightforward to determine the energy of the collisions), and therefore more challenging to analyze and less amenable to precision measurements.
The shape of the collider is also important. High energy physics colliders collect particles into bunches, and then collide the bunches together. However, only a very tiny fraction of particles in each bunch actually collide. In circular colliders, these bunches travel around a roughly circular shape in opposite directions and therefore can be collided over and over. This enables a high rate of collisions and facilitates collection of a large amount of data, which is important for precision measurements or for observing very rare decays. However, the energy of the bunches is limited due to losses fromsynchrotron radiation. In linear colliders, particles move in a straight line and therefore do not suffer from synchrotron radiation, but bunches cannot be re-used and it is therefore more challenging to collect large amounts of data.
As a circular lepton collider, LEP was well suited for precision measurements of theelectroweak interaction at energies that were not previously achievable.
Construction of the LEP was a significant undertaking. Between 1983–1988, it was the largest civil engineering project in Europe.[2]
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 theZ boson, which has a mass of 91 GeV.[2] 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 aLorentz 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 theLarge Hadron Collider (LHC).
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LEP was fed withelectrons andpositrons delivered by CERN's accelerator complex. The particles were generated and initially accelerated by theLEP Pre-Injector, and further accelerated to nearly the speed of light by theProton Synchrotron and theSuper Proton Synchrotron. From there, they were injected into the LEP ring.
As in allring colliders, the LEP's ring consisted of manymagnets which forced thecharged particles into a circulartrajectory (so that they stay inside the ring),RF accelerators whichaccelerated the particles withradio frequency waves, andquadrupoles that focussed the particle beam (i.e. keep the particles together). The function of the accelerators was to increase the particles' energies so that heavy particles can be created when the particles collide. When the particles were accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch were made to collide with each other at one of the collision points of the detector. When an electron and a positron collide, theyannihilate to avirtual particle, either aphoton or aZ boson. The virtual particle almost immediatelydecays into other elementary particles, which are then detected by hugeparticle 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 theirenergy,momentum and charge, thus allowing physicists to infer the particle reaction that had happened and theelementary particles involved. By performingstatistical analysis of this data, knowledge aboutelementary particle physics is gained. The four detectors of LEP were called Aleph, Delphi, Opal, and L3. They were built differently to allow forcomplementary experiments.
ALEPH stands forApparatus forLEPpHysics at CERN. The detector determined the mass of theW-boson andZ-boson to within one part in a thousand. The number of families of particles with light neutrinos was determined to be2.982±0.013, which is consistent with theStandard Model value of 3. The running of thequantum chromodynamics (QCD)coupling constant was measured at various energies and found to run in accordance withperturbative calculations in QCD.[3]
DELPHI stands forDEtector withLepton,Photon andHadronIdentification.[4] The DELPHI detector was known for its innovative technologies. For example, it had the world largest superconducting coil at the time, and it was the first detector which had a Ring Imaging Cherenkov (RICH) detector for particle identification. DELPHI was the first HEP experiment which used neural networks for data analysis.[5] In 2024, the collaboration released their full data set as open data.[6][7] The barrel part of the detector (without muon chambers) has been preserved in the experiment cavern and can be visited as part of the tour to LHCb.[8]
OPAL stands forOmni-PurposeApparatus forLEP. The name of the experiment was a play on words, as some of the founding members of the scientific collaboration which first proposed the design had previously worked on the JADE detector atDESY inHamburg.[9] OPAL was a general-purpose detector designed to collect a broad range of data. Its data were used to make high precision measurements of theZ boson lineshape, perform detailed tests of the Standard Model, and place limits on new physics. The detector was dismantled in 2000 to make way forLHC equipment. Thelead glass blocks from the OPAL barrelelectromagnetic calorimeter are currently being re-used in the large-angle photon veto detectors at theNA62 experiment at CERN.
L3 was another LEP experiment.[10] Its enormous octagonal magnet return yoke remained in place in the cavern and became part of theALICE detector for the LHC.
The results of the LEP experiments allowed precise values of many quantities of theStandard Model—most importantly the mass of theZ boson and theW boson (which were discovered in 1983 at an earlierCERN collider, theProton-Antiproton 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 tantalizing but inconclusive hints that theHiggs particle of a mass around 115 GeV might have been observed, a sort ofHoly Grail of currenthigh-energy physics. The run-time was extended for a few months, to no avail. The strength of the signal remained at 1.7standard deviations which translates to the 91%confidence level, much less than the confidence expected by particle physicists to claim a discovery, and was at the extreme upper edge of the detection range of the experiments with the collected LEP data. There was a proposal to extend the LEP operation by another year in order to seek confirmation, which would have delayed the start of theLHC. However, the decision was made to shut down LEP and progress with the LHC as planned.
For years, this observation was the only hint of a Higgs Boson; subsequent experiments until 2010 at theTevatron had not been sensitive enough to confirm or refute these hints.[11] Beginning in July 2012, however, theATLAS andCMS experiments atLHC presented evidence of a Higgs particle around 125 GeV,[12] and strongly excluded the 115 GeV region.
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