ZEPLIN and LUX
H. Araujo, J. Quenby, T. Sumner
The ZEPLIN programme at Boulby
The Imperial HEP group was involved in the ZEPLIN programme at the Boulby mine, UK, which pioneered dark matter searches using the extremely sensitive two-phase xenon technology. In these detectors, a particle interacting in the liquid xenon produces prompt scintillation photons and releases ionisation electrons. Using an external electric field, the free electrons can be drifted upwards to the liquid surface and emitted into a thin gas layer, where they produce more photons through electroluminescence. Both light pulses are detected with photomultiplier tubes. This time projection chamber configuration provides sensitivity down to a few scintillation photons and to single electrons emitted from the liquid; it allows very low energy interactions to be localised in 3D with millimetre accuracy; and it enables discrimination between different types of interaction, which is essential to reject the dominant backgrounds from radioactivity.
Imperial led the ZEPLIN-III experiment, the most sensitive in the ZEPLIN series, which was deployed underground in late 2006 and concluded in 2011/12. ZEPLIN-III achieved the highest level of background discrimination in these detectors and published very competitive cross-section limits for several hypothetical WIMP interactions. The ZEPLIN-III collaboration included also the Rutherford Appleton Laboratory and Edinburgh University in the UK, LIP-Coimbra in Portugal and ITEP-Moscow in Russia.
LUX: the Large Underground Xenon experiment
Since 2012 the group has been involved in the LUX experiment at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. LUX operated in the refurbished Davis Complex at the 4,850-ft level, where Ray Davis conducted his famous solar neutrino experiment from the late 1960s.
The detector contained 250 kg of active liquid xenon in its two-phase Time Projection Chamber, and published world-leading WIMP results from two main science runs, in 2013 and 2014-16. In the process, the collaboration developed key calibration techniques that have improved the quality and sensitivity of these searches very significantly. LUX completed science operations in 2016.
In parallel, the LUX and ZEPLIN teams came together in 2012 to design a multi-tonne liquid xenon experiment for SURF. The LZ experiment will have enough sensitivity to discover WIMPs if these conform to the predictions of the most popular theories of physics beyond the standard model, or to study their properties in case of a prior discovery.