A US team that claims to have built the worlds most sensitive dark matter detector has completed its first data run without seeing any sign of the stuff.
In a webcast presentation today at the Sanford Underground Laboratory in Lead, South Dakota, physicists working on the Large Underground Xenon (LUX) experiment said they had seen nothing statistically compelling in 110 days of data-taking. We find absolutely no events consistent with any kind of dark matter, says LUX co-spokesman Rick Gaitskell, a physicist at Brown University in Providence, Rhode Island.
Physicists know from astronomical observations that 85% of the Universes matter is dark, making itself known only through its gravitational pull on conventional matter. Some think it may also engage in weak but detectable collisions with ordinary matter, and several direct detection experiments have reported tantalizing hints of these candidate dark matter particles, known as WIMPs (Weakly Interacting Massive Particles). Gaitskell says that it is now overwhelmingly likely that earlier sightings were statistical fluctuations.
LUX contains more than 300 kilograms of liquid xenon held 1,480 metres underground at the Sanford lab, where rock shields it from the confounding effects of cosmic rays striking Earth's surface. The hope is that dark matter particles, passing through Earth, will occasionally hit xenon nuclei, causing flashes of light that can be picked up by the experiments 122 photomultiplier tubes, situated in arrays above and below the xenon. But only 160 events were seen, a level consistent with background levels of radioactivity from the materials making up the experiment, and well short of what would be expected if candidate dark matter particles seen by previous experiments were real, says Gaitskell.
