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LEP, the world’s biggest particle accelerator, swung into full action
last weekend. On Sunday night, only four weeks after the Large Electron
Positron collider was turned on, the machine produced its first Z0 particles.
In the autumn, LEP, which is situated at CERN, the European Laboratory for
Particle Physics, near Geneva, will become a veritable factory for producing
Z0 particles. The flood of these particles will allow physicists to make
precise tests of the theory that links two of nature’s fundamental forces,
the electromagnetic force and the weak nuclear force.

LEP occupies a circular tunnel 27 kilometres in circumference. Work
on the project began in 1981 (see ‘The making of a particle machine’, New
Å®ÉúСÊÓƵ, 8 July). The tunnel contains some 5300 magnets to guide bunches
of electrons and positrons in opposite directions around the machine. In
May, Herwig Schopper and the present director of CERN, Carlo Rubbia, installed
the last of these magnets.

The pace of progress on LEP has been outstanding. In mid-June, technicians
inserted the final sections of the pipe, in which the electrons and positrons
travel in an ultra-high vacuum to minimise unwanted collisions with air
molecules. After that, it was only a month before positrons made the first
complete orbit of LEP, on Bastille Day, as promised. (Most of LEP lies under
France.) Electrons made their first orbits around the machine 11 days later.

The electrons and positrons pass through a complex network of smaller,
older accelerators, before they enter LEP at an energy of 20 gigaelectronvolts
(GeV). LEP’s function is to accept as many electrons and positrons as possible
and then to accelerate them to as much as 50 GeV. Once at this energy, the
beams of particles coast around the machine, crossing at four points where
large experiments lie in wait to detect the sprays of subatomic particles
produced as the electrons and positrons collide.

Since the first successful orbits of LEP, the challenge to the physicists
and engineers in charge of the machine has been to increase the numbers
of electrons and positrons in the beams, and to accelerate the particles
that they produce.

With the eyes of the world on LEP, there has been intense pressure on
the accelerator experts, and on the physicists preparing the four large
experiments that observe the collisions. That LEP produced collisions at
45.5 GeV per beam – which are necessary to create the Z0 particle with a
mass of 91 GeV – by mid-August is no mean feat.

To date, the experiments have accumulated only a few Z0 particles. This
compares with 500 from the world’s highest-energy proton-antiproton collider,
at Fermilab in Illinois, and 250 from the new electron-positron machine
at the Stanford Linear Accelerator Center in California. But the researchers
believe that by September, LEP’s ability to produce Z0s will far surpass
those of the other machiens.

Å®ÉúСÊÓƵs at CERN discovered the Z0 in 1984, in work that won the current
director a share in the Nobel prize for physics. Before LEP started work,
CERN’s previous total of Z0s stood at only 50. Researchers observed these
in the collisions of protons and antiprotons, in the experiments that discovered
both the Z0 and its charged partner, the W particle. With LEP, the physicists
have the advantage of being able to ‘tune’ their machine so that the collisions
occur with exactly the right energy to create the Z0. The aim is to produce
100 000 Z0s by Christmas, a total that should be enough for physicists to
answer at least one long-standing question: ‘How many types of neutrino
are there?’. The answer lies in the intrinsic uncertainty, or ‘width’, of
the mass of the Z0. With 100 000 Z0s under its belt, CERN should have a
precise measurement of this width.