ZEPLIN-III WIMP SEARCHES WITH
TWO-PHASE XENON
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INTERNAL Ï |
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, the STFC 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). LUX operated until early 2017, producing a series of world-leading
WIMP search results along the way.
Together, they are developing
the next-generation LUX-ZEPLIN (LZ) experiment,
which will feature a multi-tonne liquid xenon target to follow LUX at SURF. LZ
is expected to begin science operations in 2020.
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ZEPLIN-III COLLABORATION 2007-2012