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The LEP story

Geneva, 9 October 2000. The LEP story begins in the late 1970s when CERN1 Member State physicists got together to discuss the long term future of high energy physics in Europe. A new picture of fundamental interactions, unifying the electromagnetic and weak forces, was emerging, and LEP would be the machine to study it. After a history built on proton machines, the idea of an electronÐpositron collider was a break with tradition for CERN. But since electrons and positrons are, as far as we know, pointÐlike particles, the results of collisions between them are far easier to interpret than the collisions between protons and antiprotons which were on CERN's immediate horizon. LEP was formally approved in 1981 and civil engineering work began on 13 September 1983 when the Presidents of CERN's two host countries, François Mitterrand for France and Pierre Aubert for Switzerland, symbolically broke the ground and laid a plaque commemorating the inauguration.

An early breakthrough

Civil engineering for LEP was a major undertaking. Although much of the necessary infrastructure for the new accelerator was already in place Ð CERN's existing accelerator complex was to pre-accelerate LEP's electrons and positrons Ð many new facilities were needed. The most obvious of these is the 27 kilometre tunnel which houses the machine, along with the experimental halls and surface buildings. Transfer tunnels joining the SPS accelerator to LEP were also needed, as were buildings to house a linear accelerator (linac) and storage rings to make and accumulate electrons and positrons. Progress was impressive. By the end of 1984 the buildings for the linac and the electronÐpositron accumulator were complete and ten of the eighteen access shafts had been excavated. On 8 February 1988 the two ends of the 27 kilometre ring came together with just one centimetre of error. Beam was injected into the first 2.5 kilometres of the ring later that year, and the first beam circulated on 14 July 1989. Collisions came one month later on 13 August, just five years, eleven months after the ground breaking ceremony.

Surveying the ground

A 27 kilometre tunnel requires careful survey work if the ends are to meet up, particularly when the tunnel is to contain a particle accelerator whose components must be aligned to within a fraction of a millimetre. At the beginning of the project, surface surveys were used to map out the surface network and tunnelling machines were then gyroscopically steered underground. The first 1300-metre section was completed in 1984 with an error in both horizontal and vertical planes of one centimetre. One year later, experience gained with the tunnelling machines led to the four kilometre arc from point 1 (at the CERN Meyrin site) to point 2 (home of the L3 experiment) being excavated with just four millimetres of error. Towards the end of 1984, terrestrial measurements were supplemented by measurements using the NAVSTAR satellite system. Using high precision time signals emitted from an onboard atomic clock, ground receivers calculated the positions of pairs of survey stations. The NAVSTAR results pinpointed eight LEP surface sites to within four millimetres of the terrestrial measurements.

Installing the detectors

The four collaborations building experiments for LEP broke their own new ground in a figurative sense becoming the most cosmopolitan groupings of physicists the world had ever seen. Detector components were built as far from CERN's Member States as China, Japan, and the USA. When the experimental halls were ready in 1988, all the various detector components were brought from around the world for final assembly at CERN. The superconducting coil of the DELPHI experiment picked a very careful 1600 kilometre route by road, ship, and barge on its journey to CERN, descending from the Jura mountains in October 1987. DELPHI's coil was the largest superconducting magnet ever built with a length of 7.4 metres, a diameter of 6.2 metres, and weighing nearly 84 tonnes. It was installed along with detector components from Belgium, Denmark, Italy, Finland, France, Germany, Greece, the Netherlands, Poland, Russia, Sweden, and the United Kingdom. Each of the other experiments - ALEPH, L3, and OPAL - has a similar story of international collaboration to tell.

Installing the accelerator

By the end of 1987, LEP magnets were stocked and ready to be installed. The white magnets in the background are LEP's innovative dipole magnets. They are made of plates of steel with the intervening spaces filled out with concrete. For the relatively low bending fields used in LEP, this technique offers a much cheaper alternative to solid steel, costing about half the price. The blue magnets in the foreground are quadrupole focusing magnets and the small yellow magnets in the background are sextupoles that correct the beams 'chromaticity'; just as optical systems correct for the different wavelengths which make up light, these sextupoles correct for the spread of momenta in LEP's particle beams. LEP's full complement of magnets runs to 3368 dipoles, 816 quadrupoles, 504 sextupoles, and 700 other magnets which apply small corrections to the beam orbits. Around each of the experiments, very high field superconducting quadrupoles are used to give the beams a final squeeze before they are brought into collision.

An early event

This is one of the first electron-positron collisions to be recorded in LEP during the accelerator's five-day pilot run which began just before midnight on 13 August 1989. The OPAL experiment had the privilege to record the very first collision, at about five past midnight, but the other three experiments followed soon after. LEP was designed to study W and Z particles - carriers of the weak force, which plays an important role in radioactivity and in the nuclear processes through which the Sun burns. In this image a Z particle disintegrates into quarks. As the quarks fly away from the collision point they generate 'jets' of particles, which are shown here. The lines show the particles' tracks, the boxes the energy they have deposited. The first fully-fledged physics run began on 20 September and continued for three months. During this time, the experiments each recorded around 30 000 Z particles, enough for LEP's first analyses to get under way.

The LEP inauguration

LEP was officially inaugurated on 13 November 1989 in the presence of some 1500 guests including Heads of State and Ministers from all of CERN's 14 Member States. Eleven years later, CERN's membership had grown to 20 states, with the addition of Finland, the Czech and Slovak Republics, Hungary, Poland, and Bulgaria. LEP results were not long in coming. By the time of the inauguration, the first results had already been announced: LEP experiments had shown that three and only three families of particles exist.

The first results

This graph presents the evidence. The total cross-section for electron-positron collisions was plotted against energy as LEP scanned through the mass of the Z particle. The result is a clear peak corresponding to visible Z-particle decays. But the Z can also decay into neutrinos which escape the detectors unseen. Shown on this plot along with the data are predictions from the Standard Model in which two, three, and four kinds of neutrinos have been assumed. The three neutrino curve clearly matches the data far better then the other two curves. All everyday matter is made of particles from the lightest family of particles. Understanding why nature has provided two heavier copies of this family is a question for a new generation of physics experiments.

The four detectors

Each LEP detector consists of layers of sub-detectors that pick up the particles produced in electron-positron collisions. The pipe carrying the oppositely-directed electron and positron beams threads through the centre of these detectors. The surrounding sub-detectors form concentric cylinders with 'end caps' plugging the ends to ensure that few particles escape detection. Each layer performs a specialised task in identifying the particles produced in the collisions.


The largest of the LEP detectors is L3. Its magnet (painted red) is made of some 8500 tonnes of iron. It measures 15.8 metres high by 16 metres long, and surrounds all of L3's sensitive elements - chambers to record the tracks of particles and calorimeters to measure their energies. Part of L3's calorimeter system is made from crystals of Bismuth Germanium Oxide (BGO). These emit light when energetic charged particles pass through them. This property of BGO is attractive to medical science, where crystals can be used in PET scanning machines. Their use in L3 pioneered their application in hospital scanners around the world.


An end-on view of a collision inside the ALEPH detector looks like this after computer reconstruction. The tracks show the passage of particles produced when an electron and a positron 'annihilated' to produce a Z particle. The Z then decayed producing the three clear jets of particles measured by ALEPH's tracking detectors and curved by the field of a superconducting electromagnet. The two outer rings on the display (red) depict the energy detected in the calorimeters. The appearance of three jets indicates that the Z particle decayed swiftly into a quark and an antiquark, but that one of these also radiated a gluon - the carrier of the strong force that holds quarks together. The total energy contained in these three jets adds up to the energy equivalent to the mass of the Z particle: 91 gigaelectronvolts (GeV).


During routine maintenance, the OPAL detector displays one of its principle sub-detectors. The electromagnetic calorimeter measures the energy of electrons and photons. It consists of thousands of blocks of lead-glass, wrapped in black to keep out unwanted light; there are 4720 blocks in this C-shaped half of the cylinder alone, and they are tapered in 16 different ways so that they all point to the place where the beams collide. To left and right, parts of the end caps are visible, revealing on the left the layered structure of the hadron calorimeter. This is mainly iron, which stops protons, neutrons, and other hadrons (particles built from quarks). It is interspersed with detectors that measure the energy these particles deposit. The iron also forms part of the electromagnet that bends the tracks of particles to provide vital information on momentum and hence energy.


After seven years of measuring the Z particle to high precision, the LEP experiments turned their attention to W particles. This computer reconstruction of particle tracks in the DELPHI detector reveals the first recorded example of the decay of two W particles produced together in an electron-positron collision at LEP. In this collision, the electron and positron have annihilated and their energy has turned into the mass of the two W particles - one positively charged, the other negative - which almost instantly decay to other particles. There are several ways that the W can decay, but in this instance, each W particle has decayed to a quark and an antiquark producing a total of four jets in the detector. This event was recorded on 9 July 1996, shortly after LEP's energy had been increased to 161 GeV per beam - just enough to make the mass of the pair of W particles.

LEP from the air

An aerial view of LEP reveals the extent of the world's largest scientific instrument. The LEP ring is marked by the white circle, with the dottedline marking the border between France towards the top and Switzerland towards the bottom. The main site of the CERN laboratory is close to where the LEP ring crosses the border in the upper left-hand corner. To the lower left, Geneva's Cointrin International Airport gives a feel for scale. LEP's four detectors are evenly spaced around the ring at locations marked by white spots. Working clockwise, ALEPH is at the top, followed by OPAL, DELPHI, and L3.

Inside the tunnel

LEP's main engines are superconducting accelerating 'cavities' like these. They serve a dual purpose - to accelerate the particle beams up to collision energy, and to maintain that energy once it has been achieved. On each circuit of the machine, particles pass through accelerating 'cavities' where they gain energy by traversing strong electric fields. When LEP started up in 1989, 128 copper accelerating cavities provided its energy. With an accelerating voltage of 300 megavolts (MV) per lap, they provided enough power to take the energy of each beam to 50 GeV - sufficient to produce Z particles. Even when LEP was still on the drawing board, a far-sighted programme of R&D into superconducting cavities was put in place to allow LEP to reach higher energies. These new cavities were installed from 1996 onwards and by 1998, a total of 272 superconducting cavities provided sufficient power - with an accelerating voltage of 2700 MV per lap - for LEP to reach a total collision energy of 189 GeV.

LEP in 2000

By May 1999, LEP's final sixteen superconducting cavities had been installed bringing the machine's energy reach to 192 GeV. But there was better yet to come. LEP's engineers decided to push the superconducting cavities through their designed operation point of 6 MV per metre to as much as 7 MV per metre, 16% beyond their nominal limit. On 2 August, right on cue for LEP's tenth birthday, their efforts were rewarded as the accelerator's first 100 GeV beams were brought into collision. Later in the year, on 25 September, the energy was taken up another notch to 101 GeV per beam and a large fraction of the last month of running in 1999 was at that energy.

Homing in on Higgs

This set the scene for a nail-biting climax to LEP's career in 2000. Eight old copper cavities were pressed back into service and the superconducting cavities have been pushed still harder. The result is that LEP has achieved a collision energy of over 104 GeV per beam and the experiments have been reporting tantalising hints of new physics. The so-far elusive Higgs particle is one of the most important missing links in physics. Its discovery could help explain the masses of the fundamental particles. LEP has already passed the most likely energy range for finding the Higgs, so each new increment brings new hope of discovery. By September 2000, all four LEP experiments had recorded collisions showing potential evidence for a Higgs particle with a mass around 114 GeV. But the evidence is so far inconclusive. We already know that if the Higgs particle exists, it will be within reach of LEP's successor, the Large Hadron Collider (LHC). So whatever happens now, CERN's flagship accelerator has already demonstrated that there will be a rich harvest of physics to be reaped when the LHC comes on-stream in 2005. As we meet to celebrate LEP's achievements, the end of the story is still to be written.

Some LEP facts and figures

  • The total amount of spoil excavated in LEP's construction was 1.4 million cubic metres.
  • Construction took five years and eleven months.
  • The LEP tunnel slopes with a gradient of 1.4%.
  • The tunnel's circumference is 26,659 metres and it runs at a depth varying from 50 to 175 metres.
  • The internal diameter of the tunnel is 3.8 metres in the arcs and either 4.4 or 5.5 metres in the straight sections.
  • Electrons and positrons in LEP lap the ring over 11 200 times per second.
  • LEP's energy measurement is sensitive to the orbit of the moon, the level of water in Lac Leman, and the departure of the TGV from Geneva station.
  • The pressure inside LEP's beam pipe is some 1015 times weaker than atmospheric pressure.


The evolution of LEP's accelerating power


Accelerating cavities



Niobium film

Solid Niobium

Accelerating voltage (MV) Beam energy (GeV)







Nov 1995






June 1996






Oct. 1996






May 1997






May 1998






May 1999






Nov 1999






May 2000







1. CERN, the European Organization for Nuclear Research, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and Unesco have observer status.