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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”
(seeNovember 2000 News).

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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