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August 14 1999 BRITAIN


Physicists on track of exotic particles pick up 'Wimp' signals, says Nigel Hawkes

Answer to universal mystery down t' pit

EXPERIMENTS at the bottom of one of Europe's deepest mines, near Whitby in North Yorkshire, may have found the first hints of the exotic particles holding the universe together. Unexplained signals have been picked up by a detector designed to find Wimps - weakly interacting massive particles - that some physicists believe make up the greater part of the universe's mass.

The team responsible, from Sheffield University, Rutherford Appleton Laboratory in Oxfordshire, and Imperial College, is not claiming to have found Wimps, Dr Neil Spooner of Sheffield University told the National Astronomy Meeting in Guernsey yesterday.

The signals are not quite the same as those they are expected to produce, but are "very intriguing", he said. An independent detector, designed by a French group, has added to the excitement by finding similar events.

The signals may be the first signs of Wimps, but they could be the accidental result of contamination or an instrument problem. Until further data is gathered, no one can tell. The discovery of Wimps would be a huge prize, comparable to the discovery of the first sub-atomic particles at the end of the 19th century.

It may seem extraordinary that particles supposed to comprise up to 90 per cent of the matter in the universe should be so hard to detect. But Wimps, if they exist, pass harmlessly through other material without affecting it. Countless billions pass through our bodies every day.

The detector down Cleveland Potash's mine at Boulby is designed to detect the rare occasions when a Wimp does collide head-on with the nucleus of another atom. The detector has been put down the mine, three-quarters of a mile beneath the surface, because rock above stops cosmic ray particles that would otherwise swamp its detector.

It is immersed in a tank containing 200 tons of distilled water to absorb any gamma particles from nuclear decay in the rocks. All that can trigger the detector, the team believes, are Wimps.

During the past year about 1,000 unexplained events have been detected. They consist of bursts of photons, the particles of light, created when something collides with atoms of sodium or iodine in the detector, which is made of sodium iodide. Such impacts accelerate the atoms, which then slow down again, releasing photons - and hence scintillating - as they do so. The light can then be seen and analysed.

The odd events do not have precisely the signature expected of Wimps, or the team would already be "jumping up and down," Dr Spooner says. "We are convinced they are scintillation events. But what is causing them? The scintillation pulses are not quite as fast as we expect for Wimps, but we are not ruling anything out."

That a different detector, made by a group led by Dr Gilles Gerbier from Saclay, the French nuclear centre near Paris, and lowered down the Boulby mine, has found exactly the same events is "quite extraordinary", Dr Spooner says.

One way to clarify the position would be to try to find the direction the apparent particles are coming from. Because the Earth is part of a galaxy in motion through a sea of Wimps, this should mean that they approach us from a calculable direction. An American group is building detectors that can be put in the mine to detect if the particles arrive from that direction.

Why should anybody think the universe is permeated with invisible and untouchable particles? Dr Spooner explains that evidence comes from astronomical observations. "Galaxies like the Milky Way whirl around, but do not fly apart," he says.

"That means they must have a certain mass within them to hold them together by gravitational forces. The mass we can see, in the form of stars, is only a tenth or a hundredth of what is needed. So there must be lots of other matter we cannot see."

Some of this is ordinary matter in the form of objects such as brown dwarfs - failed stars too dim to see. But most could be Wimps.

Another 5 million grant from the Particle Physics and Astronomy Research Council will help the team to find out.


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