The particles thought to make up dark matter often are described as “elusive.” But after yet another experiment designed to detect them has come up empty, scientists may be tempted to think of them as vexing, stubborn, or downright ornery. In fact, some physicists are starting to move away from the leading explanation for dark matter—that it’s basically a bunch of WIMPs (weakly interacting massive particles).
“I think most people involved are slowly starting to look elsewhere,” says Juan Collar, a physicist at the University of Chicago who is involved with several dark matter experiments. “Perhaps not so much the experimentalists [yet], because of their heavy investment in ongoing searches, but certainly the theory and phenomenology folk.”
Dark matter appears to account for roughly five-sixths of all the matter in the universe. It produces no detectable energy, but it exerts a gravitational pull on the “normal” matter around it. Theory says that WIMPs almost never interact with normal matter, which makes them difficult to detect.
That hasn’t stopped physicists from trying. The most recent attempt was called LUX, or the Large Underground Xenon experiment, which was installed in a former gold mine in South Dakota to filter out radiation from Earth’s atmosphere. It consisted of a third of a ton of liquid xenon surrounded by 72,000 gallons of ultra-pure water. Detectors around the water tank looked for a flash of light and electrical charge that would be produced if a WIMP smacked into the nucleus of one of the xenon atoms.
On July 21, however, during a dark matter conference in the United Kingdom, project scientists reported that LUX didn’t detect a single WIMP in 332 days of observing. It had been “the most sensitive search for WIMPs in the world,” says Richard Gaitskell, a physics professor at Brown University and one of the leaders of the LUX team.
The negative result joins those from a long list of dark-matter searches. Two experiments in the Soudan mine in Minnesota, which tried to catch the signature of a WIMP with “hockey pucks” of pure germanium, also came up empty. One of those experiments has been upgraded with more detectors, while the other has been dismantled to put effort into a larger version being built in a deeper mine in Ontario.
Other WIMP detectors are still operating, including one in Italy that uses 3.5 tons of liquid xenon, making it several times more sensitive than LUX, and one in Canada that uses liquid argon. Results from both are expected sometime in the next year.
These and other experiments have ruled out the heaviest models of dark matter particles, leading physicists to consider lighter WIMPs, or particles known as axions, or other possibilities. “The rationale for WIMPs is still strong, but perhaps with less emphasis on the ‘massive’ part,” says Dan Bauer, a physicist at Fermilab and leader of one of the Soudan experiments. “There is plenty of territory yet to search, with viable theories to motivate such searches.”
To carry out those searches, scientists are planning the next generation of detectors, including LUX-ZEPLIN, an upgraded version of LUX that will use 10 tons of liquid xenon, making it 70 times more sensitive than the earlier experiment, says Gaitskell. NASA and the European and Chinese space agencies are developing satellites to look for indirect evidence of dark matter, which could shed additional light on its nature. These experiments may be the last chance for the WIMP model of dark matter.
“From a theoretical perspective, the field as a whole is giving serious thought to alternatives beyond the primary models of WIMPs and axions,” adds John Orrell, a researcher at Pacific Northwest National Laboratory and a leader of one of the Soudan experiments. “If the next generation of experiments do not find a signal in the next five years or so, it will be time to do serious thinking about our experimental approach to direct detection.”