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A matter of deep, dark significance in the salt mines of Yorkshire

By Steve Connor, Science Editor

30 April 2003

British scientists have edged ahead in the global race to detect the most elusive substance in the universe, using what are believed to be the world's most sensitive instruments.

More than 1,000 metres below ground in a Yorkshire salt mine – the deepest mine in Britain – physicists have set up a unique experiment to find the mysterious "dark matter'' of the universe.

Dark matter is used to describe the 90 per cent of the material in the universe that does not exist as stars, galaxies, black holes and other objects, but whose gravitational influence is known to stop the galaxies from breaking apart.

One theory is that this "missing mass'' consists of dead stars or other large objects that do not produce enough light to be seen by telescopes. But theoretical predictions point to a sea of subatomic particles that have permeated the universe since their creation in the Big Bang.

The instruments needed to detect these particles have to be shielded against interference from background radiation – such as cosmic rays from outer space – by hundreds of metres of solid rock.

Known as wimps – weakly interacting massive particles – these relics of the Big Bang are so unreactive that billions can harmlessly pass through each of us every second on their journey through space.

A 3.1m underground laboratory was opened yesterday at a potash mine in Boulby in an ambitious attempt to detect these particles as they pass straight through the Earth.

Professor Neil Spooner of Sheffield University, a member of the dark matter team, said: "Now we have the best results in the world. We've not detected wimps yet but we are getting closer.''

The project, funded by the Particle Physics and Astronomy Research Council, is constantly improving the sensitivity of its instruments, which are already up to 1,000 times more sensitive than early prototypes. They will be improved another thousand-fold over the next five years, said Professor Nigel Smith of the Rutherford Appleton Laboratory in Oxfordshire. "We've seen nothing with greater sensitivity anywhere else in the world."

Scientists hope that a wimp penetrating the xenon gas-filled detectors will collide with an atomic nucleus, causing it to emit a tiny flash of light. Professor Spooner compares it to playing billiards. "You don't actually see the wimp, or cue ball itself, but you see the recoil of the billiard ball as it hits,'' he said.

By putting the detectors deep underground, the scientists are increasing the possibility of seeing a collision with a wimp. "It's like looking for a needle in a haystack. What we are trying to do is get rid of the hay to give us a better chance of detecting the needles,'' Professor Spooner said. "It's quite possible that they don't exist but all the evidence points in the direction of them existing."

Several teams, notably in the US, France and Italy, are also chasing wimps using underground detectors. "At the moment, we're just in the lead in terms of sensitivity,'' Professor Spooner said. "The Italian team has already made a claim to have found a wimp but other scientists have since shown that this could not have been possible with the sensitivity of the Italian detectors."

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