A decision is expected this week on the future of a new £1 billion atom-smasher. Technology Correspondent, Christine McGourty, describes how it could reveal what happened to the debris left after the Big Bang
THE FUTURE of particle physics may be sealed at a meeting in Geneva this Friday of
the council of Cern. Members from 19 countries are to vote on whether to give formal
approval to the seven-year project to build the £1 billion Large Hadron Collider (LHC),
a massive atom-smasher that will push forward the frontiers of knowledge of what the
universe is made of and how it evolved.
Physicists at Cern will examine the debris from high-energy head-on collisions between particles circulating a 16-mile underground ring in opposite directions at speeds verging on that of light. In the cosmic soup created, it is hoped that someone at the world-leading physics laboratory will spot the elusive Higgs boson, a particle that could explain the origin of mass.
The LHC could provide clues to other mysteries keeping physicists around the world
awake at night, such as the location and nature of astronomy's missing "dark matter" and the "antimatter" problem. As far as anyone can tell, equal amounts of matter and antimatter should have been created in the Big Bang 15 billion years ago, annihilating each other and wiping out the universe in its early stages. Why they were not remains a mystery.
Particle physicists have grand ambitions and have designed a suitably grand machine with which to achieve them.
But, before any Nobel prizes can be collected, some more down-to-earth problems have to be tackled, such as how to build the giant magnets that will bend the particles into their high-speed racetrack, how to keep the whole 30,000 tonnes of apparatus cool and, crucially, how to design the microchips that will help to distinguish the odd interesting particle collision from the billions of boring ones.
Building and operating the LHC poses unprecedented technological problems. "It will be the most sophisticated and complicated scientific instrument ever built," said Dr Lyn Evans, the Welshman in charge of construction. Part of the problem is that to save money, the LHC is to be squeezed into a tunnel that already houses another atom-smasher - the Large Electron Positron Collider (Lep).
Bending the particles round the ring - two sets going in opposite directions - requires powerful magnets. In a complex space-saving design the same magnets send the particles around adjacent pipes in both directions.
Some 1,200 magnets, each 50ft long, will sit end to end in the tunnel - a total of 1,400 tonnes of niobium - titanium cable. Others, which Britain is involved in designing, redirect the particles to cause collisions.
To mimic the energy of the primeval fireball of the early universe, the energy of a head-on proton-proton collision must be extremely high. This requires magnets with a powerful field - the goal is 8.65 Tesla. Two prototypes built in Italy are already being tested at Cern. The first achieved 9.5 Tesla before "quench" - the collapse of the magnetic field.
"That's substantially higher than we need," Dr Evans said. "We also tested for quality - the homogeneity of the field. That was excellent."
THE ENERGY required to pass the current through the magnets is kept to a minimum by
cooling them to a temperature at which they become "superconducting", losing all resistance to the flow of electric current. But this poses problems. Eight cryogenic plants will be needed to pump superfluid helium around the magnets to keep them at minus 271.2OC. Four already exist and four new ones will be built.
At present, the main source of the helium is Poland (fortunately a Cern member), where
the gas occurs naturally underground.
The engineering challenge lies in building the cold compressors to make ordinary liquid
helium superfluid. This is done by reducing the pressure from atmospheric pressure to
16-thousandths of an atmosphere - the liquid boils and the vapour carries away heat,
causing the liquid to cool further. The compressors suck away the vapour to keep the
cooling process going.
Dr Evans said the cryogenic plants in operation were more efficient than any produced
before, and engineers were designing multi-stage compressors to increase efficiency
further. Analysing the debris of each collision is as much of a challenge as producing
them. A detector called Alice will analyse collisions between nuclei of lead in a bid to
study quark gluon plasma, but the main "eyes" of the LHC are two giant detectors,
Atlas and CMS, each the size of a five-storey building, in modules that fit inside each
other like a Russian doll.
Bunches ofprotons will be sweeping through each other some 40 million times each second, each crossing producing about 20 interactions. Only one in a billion of these will be a head-on quark collision, and the detector has to spot it and record it before moving on to the next collision. State-of-the-art electronics and computer equipment are needed and must be designed to function with minimal maintenance despite constant bombardment with radiation.
The first layer of the detector will measure the speed and direction of all the charged particles produced by measuring the curvature of their trajectory in the magnetic field. Next, the energy of all neutral particles is measured in a calorimeter. Photons deposit their energy quickly, making them distinguishable, for example, from protons and pions, which deposit energy more slowly.
Some particles - the muons, heavier versions of the electron - will not be absorbed and
require their own detector. The particles are made to move through a gas, causing ionisation and production of a charge which is detectable. All that remains at the end are the neutrinos, which can pass through anything, even the Earth.
The only way to spot the neutrinos is by measuring the energy levels before and after a
collision to see if there is a deficit that might be explained by their presence. Similar
calculations may put physicists on the trail of the Higgs boson.
Dr Nick Ellis, a young British scientist building the electronics for the Atlas detector, said: "The challenge is developing the electronics that will decide which interactions are worth recording. Some 99.9 per cent of them will have to be thrown away, but we want to make sure we don't throw out anything interesting.
"We need completely new ways for taking snapshots of interactions happening every
25 nanoseconds. No commercial computer can do that."
A custom integrated circuit for one of the crucial algorithms has already been developed. Ellis said it was working well. The final Atlas processor will not be ready for five years. Two to three processors will be at the heart of each detector.
LHC will break new ground in a high-speed electronics, cryogenics, supercomputing,
vacuum technology and materials science. Evans said advances at Cern in superconductivity were already finding their way into medical scanners. The cryogenics "could help solve the energy problems of the next century", with applications in fusion reactors, he said.
LHC scientists are eager to promote potential technological spin-offs, since the main goals of the project, though of scientific value, are unlikely to change lives. Whether this helps loosen the purse strings of governments' footing the bill will be clear later this week.
