First results from world's most sensitive dark matter detector
30 October 2013
After its first run of more than three months, operating a mile underground in the Black Hills of South Dakota, a new experiment named LUX has proven itself the most sensitive dark matter detector in the world.
LUX stands for Large Underground Xenon experiment. The LUX scientific collaboration includes 17 research universities and national laboratories in the United States, the UK, and Portugal.
Dr Chamkaur Ghag leads the UCL activity in the experiment. He said: "To set a world-leading sensitivity with our very first science run is a fantastic achievement and a clear demonstration that this is the right technology to be pursuing in the hunt for direct dark matter detection."
Dark matter, so far observed only by its gravitational effects on galaxies and clusters of galaxies, is the predominant form of matter in the universe.
Weakly interacting massive particles, or WIMPs - so-called because they rarely interact with ordinary matter except through gravity - are the leading theoretical candidates for dark matter. Theories and results from other experiments suggest that WIMPs could be either "high mass" or "low mass."
LUX has a peak sensitivity at a WIMP mass of 33 GeV/c2 (see below), with a sensitivity limit three times better than any previous experiment. LUX also has a sensitivity that is more than 20 times better than previous experiments for low-mass WIMPs, whose possible detection has been suggested by other experiments. Three candidate low-mass WIMP events recently reported in ultra-cold silicon detectors would have produced more than 1,600 events in LUX's much larger detector, or one every 80 minutes in the recent run. No such signals were seen.
To set a world-leading sensitivity with our very first science run is a fantastic achievement and a clear demonstration that this is the right technology to be pursuing in the hunt for direct dark matter detection.
Dr Chamkaur Ghag
In both theory and practice, collisions between WIMPs and normal matter are rare and extremely difficult to detect, especially because a constant rain of cosmic radiation from space can drown out the faint signals. That's why LUX is searching for WIMPs 4,850 feet underground in the Sanford Lab, where few cosmic ray particles can penetrate. The detector is further protected from background radiation from the surrounding rock by immersion in a tank of ultra-pure water.
At the heart of the experiment is a 6-foot-tall titanium tank filled with almost a third of a ton of liquid xenon, cooled to minus 100 degrees Celsius. If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons (light) and electrons. The electrons are drawn upward by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons.
Light detectors in the top and bottom of the tank are each capable of detecting a single photon, so the locations of the two photon signals - one at the collision point, the other at the top of the tank - can be pinpointed to within a few millimetres. The energy of the interaction can be precisely measured from the brightness of the signals.
LUX's biggest advantage as a dark matter detector is its size, a large xenon target whose outer regions further shield the interior from gamma rays and neutrons. Installed in the Sanford Lab in the summer of 2012, the experiment was filled with liquid xenon in February 2013, and its first run of three months was conducted this spring and summer, followed by intensive analysis of the data. The dark matter search will continue through the next two years.
Dr Ghag said: "There is much more to come from LUX as we enter unchartered territory and seek signal from dark matter particles in our galaxy, and the successor LZ experiment, bringing together the LUX and UK ZEPLIN programmes, will go further still. Building on this same technology LZ will confirm any hints from LUX's next - much more sensitive - science runs, or sweep the remaining field for what we hope will be the first definitive detection."
*Regarding a WIMP mass of "33 GeV/c2 ":
Physicists express the mass of subatomic particles in electron volts (eV)
divided by the speed of light squared (c2 ) A giga-electron volt (GeV) is a
billion electron volts, or about the mass of a proton.
Image: Photomultiplier tubes capable of detecting as little as a single photon of light line the top and bottom of the LUX dark matter detector. They will record the position and intensity of collisions between dark matter particles and xenon nuclei. Credit: Matt Kapust/Sanford Underground Research Facility.