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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; new 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).