The search for the missing mass of the Universe


The Universe around us is not what it appears. The stars make up less than 1% of its mass; all the gas clouds and other objects, less than 5%. 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% 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.

Images: High resolution images are available to download from http://www.pparc.ac.uk/Nw/Md/Artcl/Boulby_pictures.asp

For further information contact:

PPARC Press Office Peter Barratt (at Boulby on Tuesday April 29th) Tel: 01793 442025, Mobile: 0787 9602899 Email: peter.barratt@pparc.ac.uk

Julia Maddock (at Boulby from Sunday April 27th to April 30th) Tel: 01793 442094, Mobile: 07901 514 975 Email: julia.maddock@pparc.ac.uk

Boulby Underground Laboratory for Dark Matter Research 01287 646300 or 01287 646301 (Please use these numbers if you can not contact the person you need directly).

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

Professor Neil Spooner
Department of Physics and Astronomy
University of Sheffield
Hicks Building, Hounsfield Road, Sheffield S3 7RH
Tel: +114 222 4422
Fax: +114 272 8079
Mobile: 0786 645 8107
Email: n.spooner@sheffield.ac.uk

Dr Nigel Smith
Particle Physics Department,
CCLRC Rutherford Appleton Laboratory,
Chilton, Didcot, Oxon, OX11 0QX
Phone: + 44 (0)1235 445151
Fax: + 44 (0)1235 446733
Email: n.j.t.smith@rl.ac.uk

Professor Tim Sumner
Astrophysics Group
Blackett Laboratory
Imperial College London
Prince Consort Road
London, SW7 2BZ
Tel: +44(0)2075947552
Mobile: +44(0)7876460864
Fax: +44(0)2075843465
Email: t.sumner@imperial.ac.uk

Dr Alex Murphy
Department of Physics & Astronomy
The University of Edinburgh
James Clerk Maxwell Building
The King's Buildings, Mayfield Road
Edinburgh EH9 3JZ
Tel: + 44 (0) 131 650 5285
Email: amurphy@ph.ed.ac.uk

Background notes: UK experiments - general methodology

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.

NAIAD - NaI (sodium iodide) Advanced Array Detector NAIAD is an array of 8 sodium iodide capsules, which produce scintillation signals when a nucleus recoils.

The outcome of a neutron-nucleus interaction in NaI (sodium iodide) is very similar to that expected from a WIMP-nucleus interaction, so neutrons and gamma rays are used to calibrate the experiments so that scientists can distinguish possible WIMP signals from other sources of signal. Electron recoils can be removed from the data by measuring the decay time of the scintillation light, typically 30% slower than that caused by a neutron or WIMP.

ZEPLIN I The ZEPLIN programme (originally 'ZonEd Proportional scintillation in LIquid Noble gases') makes use of the scintillation properties of liquid xenon. Liquid xenon has a number of properties, which make it very suitable for searching for dark matter particles:

? It is an efficient scintillator, emitting UV photons when nuclei recoil through the medium. This gives a low energy threshold and hence improved sensitivity to dark matter. ? It has a large quenching factor, meaning that much of the energy of recoiling nuclei is converted into observable forms, again improving the energy threshold. ? It contains heavy nuclei giving enhanced dark matter interaction rates and hence a better chance of seeing a signal. ? It can be purified by distillation to remove radioactive contaminants. This reduces the rate of background electron recoils, which could be confused for evidence of dark matter. ? It allows discrimination between nuclear recoil signals and background electron recoils. This very important as it allows us to discover rather than just set limits on dark matter particles.

ZEPLIN-I began operating underground at Boulby during 2001. The lead shielding eliminates most of the background pulses, which result from natural radioactivity and surviving cosmic-ray particles. The liquid scintillator veto is used to reject most of the remaining background (xenon pulses 'simultaneous' with pulses in the veto are ignored). As with the NaI detectors, ZEPLIN-I uses the different time-dependence of scintillation pulses from nuclear recoils and those from photon/electron scattering to discriminate against background that eludes both shield and veto.

DRIFT - Directional Recoil Identification from Tracks DRIFT is the first experiment to be installed in the new JIF area of the laboratory and is unique because its aim is not only to detect WIMPS, but to also determine what direction they come from. The Earth is subject to a steady stream of WIMPS from space as it moves through the Galaxy, blowing from the direction of the motion. As the Earth rotates on its axis, there should be a daily modulation of the signal direction. DRIFT is the world's first experiment designed to look for this modulation.

What is JIF? Launched in 1998, the Joint Infrastructure Fund (JIF) is a 750 million partnership between the Wellcome Trust, the Office of Science and Technology, and the Higher Education Funding Council for England. The purpose of the JIF is to enhance and modernise the research infrastructure of the UK University sector. http://www.wellcome.ac.uk/en/1/biosfgjifbacori.html

Joint Infrastructure Fund award - 3,140,366 The aim is to transform the existing UK underground facilities for dark matter and neutrino studies, thereby accelerating the present world-class UK programme searching for new sub-atomic dark matter particles believed to constitute 90-99% of the Universe and boosting prospects for UK led international experiments to study fundamental properties of neutrinos.

The proposal involves: complete refurbishment of the existing UK deep site at Boulby mine, North Yorkshire, full upgrade of the main UK University laboratories and facilities involved in developing technology for detectors for Boulby and upgrade of closely related computational research programmes at the same institutes.

Overseas Collaborators

Occidental College
Prof. D. P. Snowden-Ifft
Physics Department, Occidental College, 1600 Campus Road, Los Angeles, CA
90041, USA
Email:ifft@oxy.edu
Tel: 1-323-259-2793
Fax: 1-323-259-2704
Web: http://departments.oxy.edu/physics/dan.htm

Temple University
Prof. C. J. Martoff
Barton Hall, Temple University, 1900 N. 13th St., Philadelphia, PA,
19122-6082, USA
Email: cmartoff@nimbus.temple.edu
Tel: 1-215-204-3180
Fax: 1-215-204-5652
Web: http://nimbus.temple.edu/~cmartoff/
http://www.temple.edu/physics/faculty/martoff.html

Lawrence Livermore National Laboratory
Dr. W. W. Craig,
Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA,
94550-9234, USA
Email: craig1@llnl.gov
Tel: 1-925-423-1471
Fax: 1-925-423-1243

UCLA
Prof. D. B. Cline (Group leader), Y. Seo, F. Sergiampietri, H. Wang
Department of Physics & Astronomy, UCLA, Box 951547, Los Angeles, CA,
90095-1547,
USA
Email: dcline@physics.ucla.edu
Tel: 1-310- 825-1673

Hanguo Wang: Hanguo.Wang@cern.ch
Youngho Seo: yseo@physics.ucla.edu
Franco Sergiampietri: Franco.Sergiampietri@cern.ch
Web: http://www.physics.ucla.edu/wimps/

Texas A&M University
Prof. J. T. White
Department of Physics, Texas A&M University, College
Station, Texas 77843, USA
Email: white@physics.tamu.edu
Tel: 1-979-845-5490
Web: http://www.physics.tamu.edu/people/person.html/White,James

Institute of Theoretical and Experimental Physics
Dr. D. Akimov,
Institute of Theoretical and Experimental Physics (ITEP), Bolshaya
Cheremushkinskaya
Street 25, Moscow, 117259, Russia
Email: akimov_d@vitep1.itep.ru

LIP-Coimbra
Prof. A. Policarpo
Departamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal
Email: policarpo@lipc.fis.uc.pt
Tel: + 351 239.833.465
Fax: +351 239.822.358

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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