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.