In the late 1970s physicists from CERN Member States were discussing the long-term future of high-energy physics in Europe. A new picture of fundamental processes – unification – was emerging and the Large Electron–Positron collider (LEP) would be the machine to study it. After a history built on proton accelerators, the idea of an electron–positron collider was a break with tradition for CERN. But since the results of electron and positron collisions are far easier to interpret than collisions between protons and antiprotons (which were on CERN’s more immediate horizon with the UA1 and UA2 experiments), the LEP proposal won through. Presenting the project to CERN council, CERN Director-General Herwig Schopper reviewed the scientific justifications, budget and construction timetable for LEP. He concluded that: Very rarely in the past has there been so much unanimity and so much consensus amongst the European scientific community on the validity of a research instrument. The accelerator was formally approved on 22 May 1981.

The presidents of CERN’s two host countries, François Mitterrand of France and Pierre Aubert of Switzerland, symbolically broke the ground and laid a plaque commemorating the inauguration of the Large Electron–Positron collider (LEP) on 13 September 1983. Although much of the necessary infrastructure for the new accelerator was already in place (such as CERN’s existing accelerator complex to pre-accelerate the electrons and positrons for LEP), many new facilities were needed. The most obvious of these was the 27-kilometre tunnel that housed the machine, along with the experimental halls and surface buildings. Transfer tunnels joining the Super Proton Synchrotron to LEP were also needed, as were buildings to house a linear accelerator (linac) and storage rings to make and accumulate electrons and positrons. Despite the huge scale of the undertaking, progress was impressive. By the end of 1984, the buildings for the linac and the electron–positron accumulator were complete and ten of the 18 access shafts had been excavated.

The excavation of the tunnel for the Large Electron–Positron Collider – Europe’s largest civil-engineering project prior to the Channel Tunnel – was completed on 8 February 1988. The two ends of the 27-kilometre ring came together with just one centimetre of error. The picture above shows a tunneling crew after completing a section of the tunnel between points 2 and 3 on the LEP ring. 

With its 27-kilometre circumference, the Large Electron–Positron (LEP) collider was – and still is – the largest electron–positron accelerator ever built. LEP consisted of 5176 magnets and 128 accelerating cavities. CERN’s accelerator complex provided the particles and four enormous detectors, ALEPH, DELPHI, L3 and OPAL, observed the collisions. LEP was commissioned in July 1989 and the first beam circulated in the collider on 14 July. The picture above shows physicists grouped around a screen in the LEP control room at the moment of start-up. Carlo Rubbia, Director-General of CERN at the time, is in the centre and former Director-General Herwig Schopper is on his left. For seven years, the accelerator operated at 100 GeV, producing 17 million Z particles, uncharged carriers of the weak force. It was then upgraded for a second operation phase, with as many as 288 superconducting accelerating cavities added to double the energy and produce W bosons, also carriers of the weak force. LEP collider energy eventually topped 209 GeV in the year 2000. During 11 years of research, LEP and its experiments provided a detailed study of the electroweak interaction based on solid experimental foundations. Measurements performed at LEP also proved that there are three – and only three – generations of particles of matter. LEP was closed down on 2 November 2000 to make way for the construction of the LHC in the same tunnel.

The OPAL experiment recorded the very first collision at 23:17 on 13 August 1989 and the other three experiments followed soon after.

The first full-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 the first data analysis to get under way.  

The four collaborations working on the LEP experiments presented their first results at a seminar organised at CERN. All of them confirmed that only three types of neutrinos exist, and hence there are only three generations of matter particles.

(From left) Princess Margriet of the Netherlands, King Carl Gustav of Sweden, CERN Council President Josef Rembser, President François Mitterand of France, President Jean-Pascal Delamuraz of Switzerland, and Carlo Rubbia, Director-General of CERN at the time. This photo was taken on 13 November 1989 at the inauguration of the Large Electron–Positron (LEP) collider, an event which was attended by some 1500 guests including heads of state and ministers from all of CERN's Member States.  

LEP's year was dominated by preparations for its energy upgrade from LEP1 to LEP2. A new way of operating the machine was mastered, and new accelerating cavities were installed. In December 1995, LEP took its first step toward LEP2, with an increase of energy from 90 to 140 GeV, and earning the label LEP 1.5. The reconfigured machine worked straight away, and the experiments produced new results soon after. ALEPH, one of the LEP experiments, found some unexpected results which appeared difficult to understand on the basis of known physics. Read more.

The LEP established the Standard Model of particle physics with unprecedented precision, including all its radiative corrections. These led to predictions for the masses of the top quark and Higgs boson, which were confirmed later on. After these precision measurements, the Nobel Prize in Physics 1999 was  awarded to Gerardus 't Hooft and Martinus Veltman "for elucidating the quantum structure of electroweak interactions in physics".

The Large Electron–Positron collider was shut down for the last time at 8.00 a.m. on 2 November 2000. Members of government from around the world gathered at CERN on 9 October to celebrate the achievements of LEP and its 11 years of operational life. With the tunnel now  available for work, teams began excavating the caverns to house the four big detectors on the Large Hadron Collider.