The search for the missing mass of the Universe
Posted: April 29, 2003

The Universe around us is not what it appears. The stars make up less than 1 percent of its mass; all the gas clouds and other objects, less than 5 percent. This visible matter is mere flotsam on a sea of unknown material - so called 'Dark Matter' - a descriptor which mainly serves as an expression of our great ignorance of its nature. We know little about that sea. We do know that about 90 percent of the material in the Universe must consist of this invisible 'dark matter' in order for stars to swirl round in galactic islands, for galaxies to cluster together as they do, and for the Universe to look the way it does. The mystery of the Universe's missing mass may be about to be revealed as UK astronomers fine tune their sensitive detectors situated 1100 metres beneath the North Yorkshire moors.

The Boulby Underground Laboratory for Dark Matter Research is situated in a working salt and potash mine in Boulby on the North Yorkshire coast. Here, UK scientists have installed their experiments to detect Weakly Interacting Massive Particles [WIMPs], a prime candidate for the missing mass of the Universe. The laboratory has recently benefited from a 3.1M Joint Infrastructure Award [JIF], providing enhanced underground laboratories and complementary surface facilities to create one of the world's foremost research centres for identifying and isolating the missing mass of the Universe.

Commenting on the new facility and its research programme Prof. Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council [PPARC], the UK's strategic science investment agency, said, " This is an outstanding research facility equipped with some of the world's most sensitive dark matter detectors. It is a crucial addition to the UK's resources in a research field where British scientists are playing a world-leading role - the race by physicists around the globe to discover these exotic, as yet undetected, dark matter particles. It would be a major coup for UK science if we could win the race".

Although billions of WIMPs are probably passing through us every second, they hardly interact with ordinary matter and so are extremely difficult to detect. Occasionally though they do knock into the nuclei of atoms and the experiments at Boulby are designed to detect these rare collisions.

Prof. Neil Spooner of Sheffield University, one of the university groups involved, likens detecting the elusive WIMP to playing billiards with an invisible cue ball, " You don't actually see the WIMP, or cue ball itself, but you see the recoil of the billiard ball as it hits. If we are successful in our quest then we are looking at a place in the history books. This will be one of the great discoveries of our time".

Prof. Spooner and colleagues from the Rutherford Appleton Laboratory, Imperial College and the University of Edinburgh currently employ three WIMP detectors using different materials.

In a sample of one kilogram of material, less than one WIMP a day will hit the nucleus of an atom, causing it to recoil slightly. The experiments will detect this recoil and record it.

However, because it happens so rarely, the detectors could also pick up lots of other reactions - such as cosmic rays hitting the material, or natural radiation - which is why the experiments are housed 1100 metres underground. The Earth absorbs most of the extraneous particles like cosmic rays from space, whilst the walls of the salt mine, being very low in natural radiation, provide further protection from the rocks of the Earth's crust.

The UK Dark Matter Collaboration consists of: University of Sheffield, CCLRC Rutherford Appleton Laboratory, Imperial College and the University of Edinburgh.

WIMPs rarely interact with the matter they are passing through, (less than one WIMP a day will hit a nucleus in a kilogram of material) but this is the key to detecting their presence.

When a WIMP collides with the nucleus of an atom, it will knock it backwards and the recoil energy released by the atom can be detected in one of three ways, depending on the detector material. Either there will be a slight rise in temperature (phonon based detection), or a slight electric charge is released (ionisation) or a photon of light is released (scintillation). It is possible for more than one of these effects to occur. The UK Dark Matter Collaboration (UKDMC) operates three different kinds of detector: NAIAD is a scintillation detector; DRIFT is an ionisation detector; and ZEPLIN uses both methods.

As WIMPS so rarely collide with matter, it is important to screen out as much background noise in the small signal produced by WIMP collisions as possible. The UKDMC screens out particles from space (such as cosmic rays) and from radioactive substances by installing its detectors 1100 metres underground in Boulby salt and potash mine. The salt mine has a low natural radioactivity and absorbs most of the particles coming in from space. Encasing the detectors in lead or copper 'castles' provides more protection, reducing the radiation by a factor of a million. High purity materials are used in every stage of constructing the detector and careful analysis is carried out on all signals recorded to screen out those that are caused by other particles, for example electron recoils caused by gamma rays.

The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), and the European Space Agency. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank observatory.

PPARC's Public Understanding of Science and Technology Awards Scheme funds both small local projects and national initiatives aimed at improving public understanding of its areas of science.

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