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LEP gets a stay of Higgs execution

30 October 2000

With possible signs of the elusive Higgs particle on the
horizon (October p8), on 14 September CERN decided to
extend the life of its flagship LEP electron-positron collider
until 2 November 2000.

LEP’s 11 year period of
physics research was scheduled to finish at the end of
September, to allow commencement of serious
engineering for the installation of CERN’s new Large
Hadron Collider (LHC). However, the new Higgs hints
from the LEP experiments justify this change of plan. The
construction schedule for LHC – expected to begin
operations in 2005 – will not be affected by the prolonged
LEP operation.

One of LEP’s main physics aims has
always been to search for the missing link of the Standard
Model of particle physics – the Higgs particle, which breaks
electroweak symmetry. The Higgs field pervades the
whole of space and endows particles with
mass.

During LEP’s first phase of operations from
1989 until 1995, the collision energy was set at just over
91 GeV (the mass of the Z particle, the neutral carrier of
the weak force), and searches showed that the Higgs must
be heavier than 65 GeV. From 1996, Higgs searches at
LEP continued as successive collision energy increases
reached 202 GeV in 1999, showing that the Higgs must be
heavier than 108 GeV.

In April the stage was set for
a final push. Everything was done to boost the energy of
LEP’s particle beams as high as possible. Excellent work
by CERN teams has allowed LEP to achieve collision
energies of up to 209 GeV, well beyond the original design
energy.

As experimental data started to accumulate
above 206 GeV, a number of events compatible with a
Higgs production with mass around 114-115 GeV were
reported in the combined results of the four LEP
experiments, ALEPH, DELPHI, L3 and OPAL. In these
events, a LEP electron-positron pair could produce a
back-to-back Z and Higgs particle. However, these signals
are difficult to disentangle from more common processes,
notably the production of Z and W particle
pairs.

The prolongation of the LEP running in
October 2000 is the response to this intriguing situation.
The extension, the maximum still compatible with the tight
LHC construction schedule, should effectively double the
experimental data at collision energies above 206 GeV,
allowing the candidate Higgs signal at 114 GeV to be
tested. Such light Higgs particles would be copiously
produced at the LHC.

The decision to extend LEP’s
experimental programme set the scene for a cliffhanger
finish to its career. Many eyes are scrutinizing the latest
data.

On 9-11 October, a series of major events at
CERN marked the imminent end of the LEP era. Reports
of these events will feature in the next issue.

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