Searching for the 'God particle' in a giant underground cavern

Deep below the Franco-Swiss border lies a giant cavern big enough to house Canterbury Cathedral, and grand enough to be part of the greatest scientific experiment on Earth. At the weekend, engineers began the delicate task of preparing the snow-white cavern for the giant machine that will take up residence in its subterranean home.

If all goes to plan, the Atlas instrument - five storeys high and weighing 7,000 tonnes - will lead the search for the holy grail of physics, an elusive subatomic phenomenon formally called the Higgs boson, but nicknamed the "God particle".

The theoreticians have for years predicted the existence of the Higgs but no instrument has yet been able to find it. If scientists do detect it, the discovery could answer one of the biggest questions in science; why do we, and all other matter in the Universe, exist?

Atlas and three other particle detectors form the backbone of the large hadron collider (LHC), a stupendously complex and ambitious "atom smasher" that aims to understand the wider Universe by investigating the minute forces and particles within the realm of the atomic nucleus. The LHC is to begin smashing protons - the atomic nuclei of hydrogen atoms - in April 2005. Every second, it will generate a volume of data equivalent to all the information handled by the world's telecom companies in a year.

At the heart of the LHC is a circular, underground tunnel 27 kilometres (16.8 miles) in circumference where two beams of protons travelling in opposite directions will be accelerated to intense energy levels before being deliberately slammed into one another to shake out a God particle or two.

Cern, the European organisation for nuclear research, is organising the construction of the LHC near Geneva. It is probably the most international activity outside the United Nations, involving 6,500 scientists from 500 universities in more than 80 countries.

Britain is a leading contributor to the project, paying £72m this year - nearly 17 per cent of Cern's core funding - through the Particle Physics and Astronomy Research Council.

In engineering terms, the project is among the world's most ambitious. Scientists had to freeze an underground river with liquid nitrogen to create a permafrost medium through which they could drill out the massive underground caverns.

The magnets used to accelerate the proton beams and keep them within a tiny fraction of a millimetre of their trajectory around the tunnel are the most powerful devised. To achieve this, Cern scientist used superconducting technology at 1.8 degrees above absolute zero (minus 273C). The helium-filled magnets are colder than outer space, so this the coldest place in the Universe.

The forces generated by the magnets are huge, Lyn Evans, the project leader on the LHC, said. "When these magnets are powered, there is an electromagnetic force trying to push outwards. That force is 500 tonnes per metre, equivalent to one jumbo jet per metre, and we've got to hold it together."

Each beam has an energy equivalent to 60 kilograms of TNT, and yet their diameter is about a quarter of the thickness of a human hair. The LHC must keep the course of each beam to within a thousandth of a millimetre using magnets 20 metres high.

Scientifically, the collision of protons is not of great interest. What the Cern researchers want to analyse is what happens when the sub-atomic constituents of protons - quarks - collide. "Colliding two protons is like colliding two oranges together," Dr Evans said. "Occasionally, you'd get a collision between two pips, but you'd always get pulp. The really important ones are the collisions between pips, which are very rare. So there is a massive rejection of data to get to the quark-quark collisions."

Cern scientists believe that if the Higgs boson exists, it should materialise in just one out of 10 million million collisions. So even with 800 million collisions a second, the God particle will appear only about once every day, and the LHC's detectors must be prepared to see it when it does make an appearance.

Finding the Higgs would enable scientists to explain why matter has mass. The theory is that the Universe is pervaded by an unseen and mysterious field (the Higgs field) and that atoms, and their subatomic particles, acquire their mass by interacting with this field.

Jim Virdee, a project scientist on the compact muon solenoid (CMS) detector, a sister experiment to the Atlas, said the LHC could be compared to a more familiar instrument seen in every school laboratory. "It is like a microscope when you shine a light on an object, only the light in this case is two proton beams," Dr Virdee said. "The second thing you need with a microscope is an object to look at. Here what we're trying to do is look inside the proton and to hit the quarks head on."

The search for the Higgs, Dr Virdee said, was part of a wider quest to find a single, unified explanation for all the forces of nature, whether they operated at the level of the atomic nucleus, or at the level of stars, planets and galaxies.

"This quest to get to an understanding of the unified theory is the underpinning of most of science, if not all of science," he added. "Our notions of space and time will change, our notions of fundamental forces will change, just like the revolution that took place at the beginning of the last century [with Einstein].

"So the scientific stakes of this experiment are very high. This is why when we start collecting data, two thirds of the world's high-energy physics community will be working on these experiments."

For years physicists have worked with what is called the "standard model", which accurately explains the world around us and is supported by experimental evidence. But that model of the Universe broke down at low energies, Dr Virdee said.

"Most of us believe the Higgs boson will regulate this problem," he said. But although most theorists predict the existence of the God particle, not least Professor Peter Higgs of Edinburgh University who came up with the idea 40 years ago, it is no foregone conclusion.

Dr Virdee said: "Nature may be smarter than us, and it's possible that there is some alternative mechanism that will be even more exciting from an experimenter's point of view, because we don't always like the theorists to tell us what we're going to find. We'd like to find things that nobody predicts."

Although the LHC can be compared to a microscope, it is also a time machine that can recreate the events immediately after the Big Bang, the cataclysmic event that created time and matter.

Roger Cashmore, Cern's director of research, said if the Higgs boson existed, it must have existed at the beginning of time, long before the creation of atoms and molecules on which life depended. "They would have been around and therefore you can work out how the universe would have evolved if you know what the ingredients were at that stage," Dr Cashmore added.

"The astronomers are rather limited; they can only go back about 300,000 years after the big bang. It is only at that point that electromagnetic waves, and lightwaves propagate. We can go back further."

Although Dr Cashmore and his colleagues cringe when they hear anyone talk of the God particle, they are effusive on its possible discovery.

"What the Higgs does is give you a mechanism for generating a mass. It would be spectacularly good to find the Higgs," he said. "To get the mechanism or the underlying theory how this will work would be an enormous step forward. But let's leave God out of it."