LUX, the most sensitive dark-matter detector yet, has not captured the mysterious particles.
A US team that claims to have built the world’s 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 Research Facility in Lead, South Dakota, physicists working on the Large Underground Xenon (LUX) experiment said that 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 Universe’s matter is dark, meaning it makes itself known only through its gravitational pull on conventional matter. Scientists have proposed that it may also engage in weak collisions with ordinary matter, and several direct-detection experiments have reported tantalizing hints of candidate dark-matter particles, known as weakly interacting massive particles (WIMPs) . Gaitskell says that it is now very likely that earlier sightings were statistical fluctuations.
LUX comprises 370 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 the xenon nuclei, causing flashes of light that can be picked up by the 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 the candidate dark-matter particles seen by previous experiments were real, says Gaitskell.
LUX has now set a limit on dark-matter particle interactions that is 2–5 times more stringent than its closest rival, the XENON100 experiment at Gran Sasso National Laboratory near L'Aquila, Italy. In 2011, XENON100 also reported that it had seen no evidence for dark matter, but critics said the experiment was not sensitive enough to glimpse the very low-mass dark matter particles tentatively reported by other experiments. LUX has five times the sensitivity of XENON100 in the low-mass realm, which should allay those concerns, says Gaitskell.
Despite the no-shows at XENON100 and LUX, Laura Baudis, a physicist on XENON100 at the University of Zurich in Switzerland, says that physicists are not ready to give up on the idea of WIMPs. They may simply have a lower mass, or may be more weakly interacting than originally hoped. “We have some way to go,” she says.
Baudis adds that LUX’s results may help the US$10-million experiment to win further funding. The LUX team hopes to scale up to 7 tonnes of xenon in a $30-million upgrade called LUX ZEPLIN. LUX ZEPLIN is one of three experiments under consideration for funding by the US Department of Energy. The other contenders are an upgrade to the Super Cryogenic Dark Matter Search, an array of solid-state silicon detectors based in the Soudan mine in Minnesota; and an argon-based liquid detector, called DarkSide, at Gran Sasso.
Juan Collar, a dark-matter physicist at the University of Chicago in Illinois, thinks that LUX’s solid results will boost its chances of being selected. But he says that it would be more strategic for the energy department to fund one of the other two experiments, because they would develop other detection materials; European governments and the US National Science Foundation have already agreed to upgrade XENON100 from 100 kilograms to 1,000 kilograms of xenon. More than one large-scale xenon experiment may not be necessary, Collar says. “Everyone acknowledges that what this field needs is a variety of targets.”
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Samuel Reich, E. Dark-matter experiment comes up empty-handed. Nature (2013). https://doi.org/10.1038/nature.2013.14057