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Observational
studies of the rotation of galaxies and the motion within groups of galaxies
strongly suggest the existence of a dominant amount of matter invisible at any
electromagnetic wavelength. The cosmic microwave background, the relic
radiation which carries an imprint of the universe created when the first atoms
formed, also supports the existence of such dark
matter, and suggests a non-baryonic origin. One of the favoured forms for
this "missing mass", both theoretically and observationally, is the
WIMP (Weakly Interacting Massive Particle). These cold WIMPs are expected to
scatter off ordinary nuclei of typical detector materials at a rate of one per
kg per year or even less, yielding energy depositions below 50 keV. Their
direct detection with terrestrial detectors is therefore a great experimental
challenge.
ZEPLIN-III
is a two-phase (liquid/gas) xenon detector looking for galactic WIMP dark
matter at the Boulby Underground Laboratory (Northeast England, UK), at a depth
of 1,100 m. At this depth the cosmic-ray
background is reduced by a factor of a million. The WIMP target consists of
12 kg of cold liquid xenon topped by a thin layer of xenon vapour. These
are viewed by an array of 31 photomultiplier tubes immersed in the liquid. The
detector operates at higher electric fields than other, similar systems, namely
its predecessor ZEPLIN-II, and provides high-precision reconstruction of the
interaction point in three dimensions. Together with low-background
construction, these features give ZEPLIN-III high sensitivity for direct WIMP
searches. The second phase of the experiment has now come to an end at Boulby;
final WIMP results have been announced from these data.
The
ZEPLIN-III Collaboration includes the University of Edinburgh, Rutherford
Appleton Laboratory, Imperial College London, LIP-Coimbra (Portugal) and
ITEP-Moscow (Russia).
The ZEPLIN programme at Boulby concluded in
2012; most ZEPLIN-III groups then joined US colleagues on the LUX experiment at the Sanford Underground Research Facility (South
Dakota, US). Together, they are also planning the next-generation LUX-ZEPLIN
(LZ) experiment, which will feature a multi-tonne liquid xenon target to follow
LUX.