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IT IS 8.30 on a cold grey April morning, and Geoff Arnison has a problem.
He is trying to push a vast structure, weighing 20 tonnes and measuring
10 metres in height, along a rail. It soon becomes clear that he is going
to need help. Not far away, Oegmund Runolfsson is contemplating a larger,
more complex problem. He has to put together something that resembles one
of those interlocking wooden puzzles, but it is 10 metres across and weighs
3000 tonnes.

Arnison and Runolfsson are working on Opal, a piece of apparatus that
may answer important questions in physics, such as how many types of fundamental
particles called neutrinos there are. Opal is one of four large detectors
that are being built at CERN, the European centre for research in particle
physics, near Geneva. The new detectors will intercept particles created
in the Large Electron-Positron Collider, LEP . This machine will accelerate
and collide headon electrons and positrons at energies of 50 gigaelectronvolts.
The collisions will produce showers of high-energy particles that Opal and
the other detectors will measure.

It will be ‘all systems go’ as from 14 July, Bastille Day in France,
in which most of the huge circular accelerator lies. The reputation of CERN
is such that few people doubt that LEP will produce electron-positron collisions
soon afterwards. During the past few weeks, the atmosphere surrounding the
detectors has become almost akin to the preparation for the launch of a
space probe. Each time the physicists working on Opal ‘log on’ to their
central computer, it reminds them how many days remain before the first
collisions in LEP. As with a space probe, everything must function together
at every instant. Everyone, not just the computer, is counting down.

Around 240 physicists and 50 engineers are involved with Opal, and they
come from 24 institutions spread across the world. In addition to those
who work for CERN, the team building Opal includes people from France, Germany,
Italy and Britain – countries that have been with CERN since its beginnings
in 1954. There are also Americans, Canadians, Israelis and Japanese. Their
countries are not members of CERN, but they have ‘bought’ their way into
Opal by contributing equipment and people. Leading them all is Aldo Michelini,
a quietly spoken Italian who has been at CERN for nearly 30 years. Michelini
guides rather than rules Opal with a sensitive, democratic touch. But this
is not to say that things do not get done.

For more than six years, the various groups have worked on their contributions
to the detector. By April this year, the time had come to discover whether
it could all work together. For example, nearly 10 000 blocks of lead glass,
mounted into two large C-shapes by the team from Tokyo University, would
have to fit around detectors built by the Universities of Bonn, Freiburg
and Heidelberg. Electronics designed in the Centre d’Etudes Nucleaires,
at Saclay in France, would have to work with signals from detectors built
as far away as Carleton University and the National Research Council in
Canada. And computer programs written in places from Birmingham to Tel Aviv
would have to communicate together in perfect harmony.

Opal sits in a huge underground cavern, 70 metres long, on the edge
of Bois Chatton, only just on the French side of the border with Switzerland.
Here LEP is 100 metres below ground, with access to the experimental area
along one of three shafts. Two shafts contain lifts, for people and small
items of equipment, while two heavy cranes can lower large pieces of equipment
down to the experimental hall through the third and largest shaft, 10 metres
across.

The detector, like the others built for colliding-beam machines, consists
of many layers, each with a special purpose. The layers wrap around the
vacuum pipe in which the beams collide like a giant set of Russian dolls.
The aim is to measure every particle produced in the collisions. Even the
elusive neutrinos, which will shoot through the layers of Opal as if they
were not there at all, will leave their own calling-card. The physicists
will know the energy of the colliding beams, so by measuring the energy
of all the other particles produced in a collision, they will know how much
has ‘disappeared’ with the neutrinos.

The central piece of this huge interlocking structure is shaped like
a bobbin, approximately 8 metres long and 5 metres in diameter. Although
it weighs 420 tonnes, the bobbin is largely gas. Its major components are
gas-filled chambers, for revealing the tracks of charged particles, and
the aluminium coil of an electromagnet (a solenoid) which the chambers fit
inside. The gas in these detectors has to be held at four times atmospheric
pressure, so they are enclosed in a cylindrical pressure vessel, with concave
ends. The coil provides a magnetic field to bend the tracks of charged particles
according to their momentum.

The outer layers of Opal exist in four main sections. There are two
units about 10 metres tall and 10 metres long, which are shaped like a ‘C’
in cross-section. The inner layer of each ‘C’ consists of 4720 lead glass
blocks, which absorb electrons and gamma-ray photons and convert their energy
to photons of light. The blocks, which are about 37 centimetres long by
10 centimetres square, have been machined and polished into 16 different
tapered shapes. This variety means that they can be directed at the point
where the electron and positron beams will cross.

Outside the lead glass, lies the iron that forms the ‘yoke’ of the magnet.
The iron assists in the detection of particles (hadrons, such as protons
and pions) that can penetrate the lead glass, but which become absorbed
in the iron. Detectors in slots cut into the iron measure the energy of
these particles. The layers of lead glass and iron are repeated in two smaller
units – the ‘end-plugs’, which fit into the concave hollows at the ends
of the bobbin. The ‘C’s then fit around the central bobbin and the end-plugs,
making an almost complete enclosure around the beam pipe.

The final layer in each ‘C’ is a barrel of gas-filled detectors that
reveal the tracks of muons, the only charged particles sufficiently penetrating
to pass through the coil, the lead glass and the iron. In addition, four
tall, flat detectors will pick up any muons that leave through the ends
of the barrel. Last but not least, there are ‘forward’ detectors that measure
the tracks of particles produced close to the beam line.

By mid-April, most of the components for Opal had arrived safely at
the pit. Now was the time for the ‘first magic moment’, to use Michelini’s
words. Would the various pieces fit together? For Runolfsson, an Icelander
who is a senior physicist at CERN and technical coordinator for Opal, this
would be the culmination of several years of detailed planning. His team
had decided upon the following strategy. It would first move the central
bobbin to the ‘garage position’ – 15 metres from the beam line – to which
the whole detector can withdraw whenever it is not in use. Then they would
bring the two C-pieces up in turn to check that it fit correctly against
the bobbin. The next stage would be to pull back the ‘C’s and attempt to
insert the end-plugs. They would then bring the ‘C’s in again, and move
the whole assembly to its position on the beam line.

Wednesday, 12 April: The day for the great move to begin. Arnison’s
problem is to move two of the tall muon end-chambers along their rails to
a position at the end of the pit beyond the beam line. He soon finds some
assistance and discovers that the chambers roll easily on their rails once
they have begun to move. Moving the bobbin along its rails will take longer.
The 420 tonnes moves with the aid of two hydraulic rams, which can push
it about 1 metre at a time. However, it does not move on its own. Two ‘gondolas’
follow the bobbin on overhead rails. These are cabins – named after cable
cars in ski resorts – that contain electronics to process the signals from
various detectors.

By early afternoon, the bobbin is approaching its garage position. Members
of Runolfsson’s team are located at strategic positions watching carefully
for any mishaps, for piping or cables, for example, that someone perhaps
overlooked and which might be damaged. A typical minor incident occurs when
a pipe on one of the gondolas following the bobbin hits another pipe on
a stationary gondola, at the other side of the bobbin. Someone hits the
‘panic’ button, and everything comes to a stop. But it is not too difficult
to move the stationary gondola a little. Then at nearly 14.30, the call
goes out: ‘Arretez! Stop!’. The first step is over; the bobbin is in the
garage position.

Later, Paul Murphy and colleagues from the University of Manchester
arrive to check what is happening to the set of muon chambers that will
lie under the bobbin. These chambers were built in Manchester from the largest
printed circuit boards available in the world – 1.1 by 3.3 metres. They
travelled from Britain by container to be mounted onto one framework at
CERN, before being lowered down the shaft to the bottom of Opal’s pit. The
question now is whether they fit under the bobbin.

Thursday, 13 April: At 14.00 hours, members of the Opal team who are
at CERN gather for their weekly meeting to discuss progress with the installation,
problems that may lie ahead and details of the physics they hope to study
once LEP is under way. This time there is news from the US. Michelini announces
that physicists working on the Stanford Linear Collider (SLC) – often seen
as being in direct competition with LEP – have found their first example
of a Z⁰ particle, the neutral carrier of the weak force. The SLC has travelled
a long hard road to reach so far, but this first small sign of success is
an extra incentive to the researchers at CERN to keep on their toes. Z⁰ particles
should be produced abundantly at LEP, but it would be frustrating for CERN
if SLC were to be first with the interesting discoveries.

Back in the pit, work continues. By now, the team has raised the end-plugs
to the correct height. The next step is to attach large steel hooks that
rise from each end-plug to two ‘trolleys’ on top of the bobbin. This task
involves two engineers rigged out in overalls, helmets, climbing harnesses
and training shoes. The climbing harnesses are essential for working on
top of the bobbin, nearly 12 metres above the ground. A rope strung across
the top provides the anchorage. Particle physics is not for the faint of
heart. Some of the engineers have also begun to move the C-pieces towards
the bobbin. Each ‘C’ bears three ‘rucksacks’ – huts for electronics, which
are stacked above each other on the ‘backs’ of the ‘C’s, making them resemble
huge snails. Their speed is appropriately slow – a maximum of 5 metres per
hour, pushed along rails by hydraulic rams.

Friday, 14 April: One ‘C’ is now close to the bobbin. By this stage,
progress appears very slow, but Runolfsson’s team has a complex three-dimensional
puzzle to resolve. The problem is to use the various jacks available to
manoeuvre the bobbin and the ‘C’ delicately, until the ‘C’ fits as snugly
round the bobbin as possible, without anything touching to the point of
being crushed.

Accurately machined surfaces on the steel framework of the bobbin and
the ‘C’ should eventually close together. At present a gap of about 5 centimetres
is visible between plates at the ends. The inner surface of each ‘C’ consists
of the ends of the lead glass blocks, wrapped in black plastic. The design
allowed for a gap of 20 millimetres between these blocks and the detectors
on the outer surface of the bobbin. But how big is this gap in reality,
and by how much does it vary over the large areas involved? Each block weighs
around 20 kilograms, and together the massive weight of the blocks in each
‘C’ has distorted the framework slightly.

Tomio Kobayashi and his colleagues from Japan have a keen interest in
the safety of their lead glass and the phototubes that register light emitted
by particles traversing the glass – together worth Pounds sterling 4 million.
They have installed electrical sensors that measure the gap between the
inner surface of a ‘C’ and the bobbin at several places. The engineers have
also positioned their own gauges to show movements as little as 0.1 millimetres
that occur when they make an adjustment via one of the jacks. After each
tiny manoeuvre, the team discuss how to tweak another jack to bring the
structures better into line.

Wednesday, 19 April: Five days later – and a week after ‘the move’ began
– the engineers have at last succeeded in making each ‘C’ in turn fit around
the bobbin. Now they have rolled both ‘C’s back some distance, and the time
has come to try the end-plugs. Each end-plug contains 1250 blocks of lead
glass as well as iron for the ends of the magnet yoke. Together these parts
weigh 130 tonnes. They move into the ends of the bobbin, pulled by the trolleys
to which they are now attached.

One difficulty with installing an end-plug is that once it begins to
enter the concave dome at the end of the bobbin, no one can see by eye if
anything is touching. Lutz Kopke from CERN has installed some pressure sensors
on the faces and around the sides of the end-plugs. He puts on a climbing
harness and totes his computer to the top of the bobbin, from where he has
relatively easy access to electricity and to the sensors at both ends of
the bobbin.

As the first end-plug moves slowly inwards, physicists and technicians
keep an anxious watch on their precious detectors. Ken Bell and Martin Sproston
from the Rutherford Appleton Laboratory are there, responsible for the lead-glass.
George Mikenberg, from the Weizmann Institute in Israel stands close by,
sucking nervously at his unstoked pipe, as the delicate chambers he has
installed in slots in the iron slide slowly from view. And Michelini is
there, on one of his frequent visits to check progress in the pit. By the
end of the day, the first end-plug has fitted successfully into the end
of the bobbin, with room to spare. And some of the team have begun to move
the other end-plug in towards the bobbin. Runolfsson is pleased with progress.
A hint of optimism seems to be overcoming his usual cautious pessimism.

Friday, 21 April: With both end-plugs now apparently in the correct
position, the task of bringing the ‘C’s back to close round the bobbin complete
with its end-plugs can begin. But as time goes by it becomes clear to the
engineers that one end-plug is not in quite the right position. There are
steel bolts, a metre or so long, which should hold plates at the ends of
the ‘C’s to the end-plugs. If the alignment is not right, then these bolts
will not go in properly – and that is what has happened.

The weekend is about to start, however, and the installation crews will
take their two days of rest. Kobayashi, meanwhile, is keen to test out the
lead glass in the ‘C’s with cosmic-rays – the natural ‘rain’ of the high-energy
particles. In particular, his goal is to synchronise signals from the lead
glass with those from scintillation counters that form one of the outer
layers of the bobbin. So the engineers decide to leave the detector almost
closed over the weekend; they will return to adjusting the end-plug on the
following Monday.

The collection of some data from the cosmic rays will be another important
milestone for Opal on the road to the first electron-positron collisions.
The ultimate purpose of the scintillation counters on the bobbin will be
to measure the time-of-flight of particles emerging from collisions in LEP.
But in the test with cosmic rays, the very act of collecting data simultaneously
from detectors in different portions of Opal – the ‘C’s and the bobbin –
will for the first time try out the complex hierarchy of electronics and
microcomputers that eventually records the data on magnetic tapes. On the
previous weekend, Aleph, one of the other detectors for LEP, had staged
an open day, at which computer monitors displayed ‘live’ the tracks of cosmic
rays through most of the apparatus. The people working on Opal are aware
that they still have much to do to reach that stage.

Monday, 24 April: There is an ambivalent mood about the weekend’s tests.
Kobayashi has some magnetic tape containing about two-and-a-half hours worth
of data from cosmic rays, collected on Sunday evening. But this was only
from one ‘C’ – and what is worse, Kobayashi is having difficulty reading
the tape. Only by late on Monday evening does Mashimo Tetsuro solve the
problem so that the Japanese team can settle down to analysing its data
on cosmic rays.

Meanwhile, the engineers have pulled the ‘C’s back from the bobbin,
to commence battle again with the alignment of the troublesome end-plug.
This is proving to be the biggest problem that they have so far encountered
in fitting the apparatus together. In the end, it takes the best part of
two days to make the almost imperceptible adjustments that are necessary.
But at last, all is well, and Runolfsson’s team can move the ‘C’s back in
again, and this time bolts them to the end-plugs. Hope appears on Kobayashi’s
horizon as he finds someone to help him unravel the data from his tapes.

Thursday, 27 April: At the weekly meeting, Patrick Elcombe, from Cambridge
University, reports on the previous weekend’s tests of the data acquisition
system. ‘Were they a success?’ he asks, ‘The answer is a qualified ‘yes’
. . . The tests were worthwhile because we found a lot of problems which
could be identified.’ Kobayashi points out that it was the first time that
data had gone through the whole chain to be analysed later, ‘off-line’.
And Kiyotomo Kawagoe from Tokyo presents results from his analysis of the
tape, now that the team has understood the error that occurred in writing
the data.

At the pit, after careful surveying on the previous day, the engineers
have moved the whole assembly of bobbin, ‘C’s, and end-plugs to the beam
line, where eventually LEP’s beams will cross at almost the speed of light.
Up on the surface at 16.00 hours there is wine, bread and cheese for everyone
who has been involved in bringing Opal this far. Runolfsson climbs up on
some scaffolding to present a small speech in his inimitable brand of French
with an Icelandic accent. ‘Well,’ he says, ‘it’s finally bolted together,
with nothing broken.’ The gathered crowd, mainly of engineers and labourers,
laughs. ‘But it came close,’ he adds. He thanks the whole team. ‘Now,’ he
says, ‘it’s up to the physicists.’

* * *

LEP adds a new dimension to particle physics

LEP, the Large Electron-Positron collider, is the world’s largest particle
accelerator. It occupies a tunnel 3.8 metres wide and 27 kilometres long,
which forms a ‘ring’ consisting of eight curved sections and eight straight
sections. The ring, which is mainly in France, stretches from CERN’s main
site near Geneva, out to the foothills of the Jura mountains, where it lies
as much as 180 metres below the surface. To avoid as much as possible of
the limestone in the Jura, the ring is tilted slightly, with a gradient
of 1.5 per cent, so that it is slightly higher by the Jura than it is near
Geneva.

Bunches of electrons and positrons enter the LEP from the 15-year-old
Super Proton Synchrotron, which accelerates them to an energy of 50 gigaelectronvolts.
They then travel many times round the ring, guided by 3328 bending magnets
and kept in tight pencil-like bunches by 1272 focusing magnets. As they
pass through two of the straight sections on opposite sides of the ring,
the particles cross hollow copper ‘cavities’ where they are accelerated
by radio waves with a frequency of 352 megahertz. In the first phase of
LEP, the maximum energy will be 50 gigahertz per beam, but in a second phase,
based on superconducting cavities, the energy of the beams will increase
to 100 gigahertz.

It has taken about six years for CERN to build LEP, at a cost of about
1180 million Swiss francs. Around 1300 scientists will work on four large
detectors, called Aleph, Delphi, L3 and Opal. They will complement each
other by each having strengths in different kinds of measurement.