The final curtain falls on LEP

25 January 2001


After a concerted push by physicists to extend the running of CERN’s LEP 27 km electron-positron collider into 2001, the decision has been taken to close the machine for good.

The original masterplan foresaw closure in September after 11 years of running, but unprecedented collision energies above 200 GeV enabled several of the experiments to glimpse signs of the long-awaited Higgs particle, which endows all other particles with mass. LEP was thus given a six-week “stay of Higgs execution”.

Extra evidence seen during the extension shows the tentative LEP Higgs signal to have a a mass of around 115 GeV. These candidate events are dominated by the production in LEP’s electron-positron collisions of a Higgs particle and a Z boson, although evidence for other Higgs production mechanisms is also seen. However, the combined effect falls slightly short of what is required to claim an outright Higgs discovery.

Physicists continued to push for additional LEP running, but on 17 November the committee of CERN’s governing body, Council, gave its verdict. Council expressed its “recognition and gratitude for the outstanding work done by the LEP accelerator and experimental teams. It has taken note of the request by many members of the CERN scientific community to continue LEP running into 2001 and also noted the divided views expressed in the scientific committees consulted on this subject. On the basis of these considerations and in the absence of a consensus to change the existing programme, Council supports the director-general in pursuing the existing CERN programme.”

The “existing programme” meant the plan to close LEP in 2000 and focus resources on the LHC proton collider, to be installed in the LEP tunnel and scheduled to start running in 2005.

While the Higgs evidence was compelling, the mechanisms involved were also at the extreme end of LEP’s energy reach, so the physicists could only touch the Higgs candidates with their fingertips. There were doubts that additional running would substantially consolidate the signal. This, coupled with the need to keep LHC construction on schedule, led to the final controversial decision.

It is rare that major particle accelerator machines close at CERN. The usual pattern is that new machines stand on the shoulders of their predecessors. The electrons and positrons for LEP came via a chain of more mature machines, including the 28 GeV PS synchrotron, which, when it first came into operation in 1959, was briefly the world’s highest-energy accelerator, and the 450 GeV SPS synchrotron, commissioned in 1976.

One CERN machine that closed was the laboratory’s first accelerator, the 600 MeV synchrocyclotron (SC), commissioned in 1957 and turned off in 1990. The SC was a stand-alone accelerator and did not serve as an injector for any later machine, but it did spawn the ISOLDE on-line isotope separator, subsequently transferred to the PS Booster.

Another past CERN machine was the Intersecting Storage Rings – the world’s first proton collider. It was commissioned in 1971 but was switched off early in 1984 to release resources for LEP construction.

The LEAR low-energy antiproton ring, commissioned in 1983, was terminated in 1996 to free resources for the LHC.

The 1983 groundbreaking ceremony for LEP was a major milestone in CERN’s history. LEP was the initial reason for the 27 km tunnel excavated under the Swiss-French frontier, but it was understood almost from the outset of LEP preparations in the mid-1970s that the tunnel would be a valuable piece of physics research real estate that would one day house a more powerful machine – LHC.

In a final proud gesture before the curtain came down on its part in the play, LEP, operating at high energies unforeseen until late in its career, revealed its intriguing hints of the long-awaited Higgs particle. This physics now has to await confirmation and consolidation at Fermilab’s Tevatron proton-antiproton collider and/or LHC.

The step from LEP to LHC is a natural progression. LEP’s disappearance is not an abrupt closure of a thriving machine – it gave all that was expected of it, and more.

LHC will take its particles from the Booster-PS-SPS chain of synchrotrons. As well as this physical supporting infrastructure, LHC will stand on metaphorical shoulders – the extinct ISR – for it was here that CERN first acquired collider expertise; the additional skills acquired at the SPS, which operated as the world’s first proton antiproton collider (1981-1990); and, of course, LEP.

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