Editing Large Electron–Positron Collider

{{Short description|Particle accelerator at CERN, Switzerland}}{{Redirect|LEP}}
{{Infobox particle accelerator
| name           = Large Electron–Positron Collider
| image          = Inside the CERN LHC tunnel.jpg
| caption        = The former '''LEP''' tunnel at [[CERN]] being filled with [[magnets]] for the [[Large Hadron Collider]].
| type           = [[Synchrotron]]
| beam           = [[Electron]]s, [[positron]]s
| target         = [[Collider]]
| energy         = 209 GeV
| current        = 6.2 mA
| brightness     =
| luminosity     = {{val|1e32|up=cm<sup>2</sup>⋅s}}<ref>{{cite journal |last1=Myers |first1=Steve |title=The Greatest Lepton Collider |journal=CERN Courier |date=September 11, 2019 |url=https://cerncourier.com/a/the-greatest-lepton-collider/ |access-date=22 April 2022}}</ref>
| radius         =
| circumference  = {{val|26659|u=m}}
| location       = [[Geneva, Switzerland]]
| coordinates    = {{coord|46|14|06|N|06|02|42|E|type:landmark|display=inline,title}}
| institution    = [[CERN]]
| dates          = 1989–2000
| preceded       =
| succeeded      = [[Large Hadron Collider]]
}}
The '''Large Electron–Positron Collider'''  ('''LEP''') was one of the largest [[particle accelerator]]s ever constructed. It was built at [[CERN]], a multi-national centre for research in nuclear and particle physics near [[Geneva]], Switzerland. 

LEP collided [[electron]]s with [[positron]]s at energies that reached 209 GeV. It was a circular collider with a [[circumference]] of 27 kilometres built in a tunnel roughly 100&nbsp;m (300&nbsp;ft) underground and passing through [[Switzerland]] and [[France]]. LEP was used from 1989 until 2000. Around 2001 it was dismantled to make way for the [[Large Hadron Collider]], which re-used the LEP tunnel. To date, LEP is the most powerful accelerator of [[lepton]]s ever built.

==Collider background==
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). [[Lepton]]s 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.

[[File:LEP Particle beam tube.JPG|thumb|right|A section of the LEP Particle beam tube]]
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 from [[synchrotron 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 the [[electroweak interaction]] at energies that were not previously achievable.

==History==
Construction of the LEP was a significant undertaking. Between 1983–1988, it was the largest civil engineering project in Europe.<ref name="MyersPicasso2006">{{cite journal|last1=Myers|first1=S.|last2=Picasso|first2=E.|title=The design, construction and commissioning of the CERN large Electron–Positron collider|url=https://cds.cern.ch/record/226776|journal=Contemporary Physics|volume=31|issue=6|year=2006|pages=387–403|issn=0010-7514|doi=10.1080/00107519008213789|url-access=subscription}}</ref>

When the LEP collider started operation in August 1989 it accelerated the electrons and positrons to a total energy of 45&nbsp;[[GeV]] each to enable production of the [[Z boson]], which has a mass of 91&nbsp;GeV.<ref name="MyersPicasso2006" /> The accelerator was upgraded later to enable production of a pair of W bosons, each having a mass of 80&nbsp;GeV. LEP collider energy eventually topped at 209&nbsp;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==
[[File:Radio frequency cavity-IMG 5781-white (cropped).jpg|thumb|upright=1.5|right|An old [[RF cavity]] from '''LEP''', now on display at the [[Microcosm (CERN)|Microcosm]] exhibit at [[CERN]]]]

LEP was fed with [[electron]]s and [[positron]]s delivered by CERN's accelerator complex. The particles were generated and initially accelerated by the [[LEP Pre-Injector]], and further accelerated to nearly the speed of light by the [[Proton Synchrotron]] and the [[Super Proton Synchrotron]]. From there, they were injected into the LEP ring.

As in all [[ring collider]]s, the LEP's ring consisted of many [[magnet]]s which forced the [[electric charge|charged]] particles into a circular [[trajectory]] (so that they stay inside the ring), [[Resonator#Electromagnetic|RF accelerator]]s which [[accelerate]]d the particles with [[Radio wave|radio frequency waves]], and [[quadrupole]]s 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, they [[Annihilation|annihilate]] to a [[virtual particle]], either a [[photon]] or a [[Z boson]]. The virtual particle almost immediately [[Radioactive decay|decay]]s into other elementary particles, which are then detected by huge [[particle detector]]s.

==Detectors==
<!--{{Main|List of Large Electron–Positron Collider experiments}}-->
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 [[statistics|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===

{{Main|ALEPH experiment}}

ALEPH stands for '''''A'''pparatus for '''LEP''' '''pH'''ysics 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 {{val|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 [[perturbation theory|perturbative]] calculations in QCD.<ref>{{cite web|url=http://aleph.web.cern.ch/aleph/aleph/Public.html | title=Welcome to ALEPH|access-date=2011-09-14 }}</ref>

===DELPHI===
{{Main|DELPHI experiment}}
DELPHI stands for '''''DE'''tector with '''L'''epton, '''P'''hoton and '''H'''adron '''I'''dentification''.<ref>{{cite web|url=https://delphiwww.cern.ch | title=DELPHI experiment home page| access-date=2025-04-05}}</ref> 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.<ref>{{cite web|url=https://cds.cern.ch/record/241402|title=Classification of the hadronic decays of the Z into b and c quark pairs using a neural network, Phys. Lett. B295 (1992) 383| access-date=2025-04-05}}</ref> In 2024, the collaboration released their full data set as open data.<ref>{{cite web|url=https://home.cern/news/news/computing/digital-archaeology-new-lep-data-now-available-all | title=CERN News article about DELPHI Open Data| access-date=2025-04-05}}</ref><ref>{{cite web|url=https://opendata.cern | title=CERN Open Data portal| access-date=2025-04-05}}</ref> 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.<ref>{{cite web|url=https://lhcb-outreach.web.cern.ch/visiting-lhcb-at-pit-8|title=Visiting LHCb at Pit 8|access-date=2025-04-05}}</ref>

===OPAL===
{{Main|OPAL experiment}}
OPAL stands for '''''O'''mni-'''P'''urpose '''A'''pparatus for '''L'''EP''. 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 at [[DESY]] in [[Hamburg]].<ref>{{cite web|url=http://opal.web.cern.ch/opal/ |title=The OPAL Experiment at LEP 1989–2000|access-date=2011-09-14 }}</ref> OPAL was a general-purpose detector designed to collect a broad range of data. Its data were used to make high precision measurements of the [[W and Z bosons|Z boson]] lineshape, perform detailed tests of the Standard Model, and place limits on new physics. The detector was dismantled in 2000 to make way for [[Large Hadron Collider|LHC]] equipment. The [[lead glass]] blocks from the OPAL barrel [[Calorimeter (particle physics)|electromagnetic calorimeter]] are currently being re-used in the large-angle photon veto detectors at the [[NA62 experiment]] at CERN.

===L3===
{{Main|L3 experiment}}
L3 was another LEP experiment.<ref>{{cite web|url=http://l3.web.cern.ch/l3/ |title=L3 Homepage|access-date=2011-09-14}}</ref> Its enormous octagonal magnet return yoke remained in place in the cavern and became part of the [[A Large Ion Collider Experiment|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, the [[Proton-Antiproton Collider]]) to be obtained—and so confirm the Model and put it on a solid basis of empirical data.

==Higgs boson==
Near the end of the scheduled run time, data suggested tantalizing but inconclusive hints that the [[Higgs boson|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 deviation]]s 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 the [[LHC]]. 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 the [[Tevatron]] had not been sensitive enough to confirm or refute these hints.<ref>{{cite arXiv|author=[[Collider Detector at Fermilab|CDF Collaboration]], [[D0 experiment|D0 Collaboration]], [[Tevatron|Tevatron New Physics]], [[Tevatron|Higgs Working Group]]|date=2010-06-26|class=hep-ex|eprint=1007.4587|title=Combined CDF and D0 Upper Limits on Standard Model Higgs-Boson Production with up to 6.7 fb<sup>−1</sup> of Data}}</ref> Beginning in July 2012, however, the [[ATLAS experiment|ATLAS]] and [[Compact Muon Solenoid|CMS]] experiments at [[LHC]] presented evidence of a Higgs particle around 125 GeV,<ref>{{cite web|url=http://home.web.cern.ch/about/updates/2013/03/new-results-indicate-new-particle-higgs-boson|title=New results indicate that new particle is a Higgs boson - CERN|website=home.web.cern.ch|access-date=24 April 2018|url-status=live|archive-url=https://web.archive.org/web/20151020000722/http://home.web.cern.ch/about/updates/2013/03/new-results-indicate-new-particle-higgs-boson|archive-date=20 October 2015}}</ref> and strongly excluded the 115 GeV region.

==See also==
* [[Electron–positron annihilation]]
* [[Large Hadron Collider]]

==References==
{{Reflist|30em}}

==External links==
*{{Commons category-inline}}
* [http://delphiwww.cern.ch/offline/lepwgs.html LEP Working Groups]
* [https://cds.cern.ch/record/226776 The LEP Collider from Design to Approval and Commissioning] excerpts from the [[John Adams (physicist)|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 [https://web.archive.org/web/20040714083204/http://hepwww.rl.ac.uk/pub/bigbang/part1.html this online booklet] of the British ''[[Particle Physics and Astronomy Research Council]]''.
* [https://inspirehep.net/experiments/1607855 Experiment Record for LEP] on [[INSPIRE-HEP]]

{{CERN}}
{{Authority control}}

{{DEFAULTSORT:Large Electron-Positron Collider}}
[[Category:CERN accelerators]]
[[Category:Particle physics facilities]]
[[Category:CERN facilities]]