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LEP reaps a final harvest

1 December 2000

CERN’s LEP electron-positron collider stubbornly refused to
lie down quietly in 2000. The world’s largest synchrotron
storage ring was scheduled to be closed forever at the end of
September, and dismantled to make way for the LHC proton
collider to be built in the same 27 km tunnel.

However,
with tantalizing glimpses of the long-awaited Higgs particle
appearing at the last gasp, LEP was accorded a six-week stay
of “Higgs execution” (see November
News
). The machine duly finished its 2000 run on 2
November. In a specially convened meeting of the LEP
Experiments Committee on 3 November, LEP physicists
revealed the fruit of these extra few weeks of autumn
running.

The Higgs particle, which breaks electroweak
symmetry and endows particles with mass, is the missing link
in the Standard Model of particle physics, and a major
objective at LEP. As LEP’s energy was increased over the
years, more and more Higgs territory has been covered
without finding any signs of the elusive particle – until this
year.

To boost the energy of LEP’s particles, from
1996 the machine was equipped with superconducting
radiofrequency accelerating cavities. The remarkable success
of this scheme, together with astute planning and skilled
machine operations, have enabled LEP to reach collision
energies of up to 209 GeV, beyond its planned energy
horizon.

For the past several years, LEP has been
running in exactly the energy band where the Higgs had been
most expected. Each time the energy was increased,
physicists held their breath. As data started to accumulate
above 206 GeV late this summer, a few electron-positron
events suggested Higgs production with a mass of around
114-115 GeV.

In these events, a LEP electron-positron
pair could produce a Higgs back-to-back with another
particle. However, the Higgs signals are right at the extreme
edge of LEP’s kinematic reach, and are difficult to disentangle
from more common processes, notably the production of Z
and W particle pairs.

The particles can decay in a
number of ways. The initial candidates saw four confined
sprays (“jets”) of particles, two from the Higgs. However,
other decay patterns are possible, and the recent run has also
revealed events with two slices of “missing mass”, indicating
the production of two otherwise invisible neutrinos, and other
signals.

In the combined results of the four LEP
experiments – ALEPH, DELPHI, L3 and OPAL – confidence
in the candidate Higgs signal therefore slightly increased as a
result of the autumn run, but still fell short of the level needed
to claim a physics discovery. The experiments therefore
requested a further extension of LEP running in
2001.

However, with the LHC knocking loudly on the
door, this has been ruled out. LEP has run for the last time,
and its ultimate findings point the way to future physics at the
LHC.

At the 3 November meeting where the latest
LEP results were disclosed, there was an ovation for the LEP
operations team which had delivered the high-energy goods
and provided such a cliffhanger finish to the machine’s 11
year career (see LEPilogue:
marking the end of an era
).

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