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.