The LUX Dark Matter Search Experiment
The Large Underground Xenon (LUX) experiment is a 350 kg liquid xenon time-projection chamber that aims to directly detect galactic dark matter in an underground laboratory 1 mile under the Earth's surface at the Sanford Underground Research Facility, in the Black Hills of South Dakota, USA.
The basics of a liquid xenon time projection chamber are illustrated by the animation below. As WIMPs stream through the Earth, if one were to interact within LUX it would produce an initial flash of scintillation light (S1) shortly after it scattered from a Xe nucleus. This would be followed by electrons being drifted away from the interaction site and into a thin layer of gas above the liquid, where another flash of light is produced. Both the drifting of the electrons into the gas and the production of this secondary scintillation or electroluminescence (S2) is courtesy of an electric field applied across the xenon between grid electrodes. Once generated, the light is detected by two arrays of photomultiplier tubes. The hit pattern of the light allows reconstruction of two of the dimensions for the interaction vertex, and the time between the two flashes gives the third. The ability to fiducialise the xenon volume, easily rejecting backgrounds that appear on the edge of the xenon, is one of the key features of such detectors. Where WIMPs will have no problem penetrating the xenon target and producing scatters uniformly, external radiation will preferentially be stopped by the outer layer of xenon, leaving a clean central volume in which to seek WIMPs.
In addition to position reconstruction, the two signals provide a powerful particle discriminant. The ratio of size of the two signals depends on the interacting species. A background gamma-ray, for example, might Compton scatter off an orbital electron, whereas a WIMP or neutron would scatter off the atomic nucleus. The power of the technology is that recoiling nuclei appear distinct from recoiling electrons. WIMP scatters would produce events with large S1 signals but small S2's, whereas the converse is true for gamma-rays or electron recoils. As a consequence, most background events can easily be rejected to increase the signal to noise ratio.
Clicking on the second image will show pictures from the construction and installation of the LUX detector. Any material used in LUX had to pass stringent radioactivity contraints so as not to introduce background and compromise sensitivity to WIMPs. Similarly, all internals have been designed such that any light generated is collected efficienctly by the photomultiplier tubes. The vessel holding the inner time projection chamber is made from ultra-low background titanium, with the entire enclosure surrounded by several metres of water (itself instrumented with photomultipliers) to shield from and identify external backgrounds that might produce a signal in LUX.
The last image shows some of the collaboration members at a recent meeting in Lead, SD, in April 2013. The collaboration boasts considerable Dark Matter search experience, with many members amongst the pioneers of dual phase noble gas technology for WIMP searches.
LUX began accruing Dark Matter search data in 2013 to become the world's most advanced instrument in the hunt for WIMPs within only a few months of operation. Results from its first exposure set world-leading sensitivity and the most stringest constraints on spin-independent WIMP-nucleon interactions to-date. LUX will now conduct a WIMP search run with at least five times more sensivity, reaching into previously un-probed parameter space, in search of the first robust detection of galactic Dark Matter.