New Experiment Torpedoes Lightweight Dark Matter Particles

If a WIMP strikes a xenon nucleus with the LUX detector, the recoiling nucleus should produce telltale flashes of light.

Flashy. If a WIMP strikes a xenon nucleus with the LUX detector, the recoiling nucleus should produce telltale flashes of light.

Carlos Faham/LUX Dark Matter Experiment

It’s not the result physicists were hoping for, but data from a new experiment may put an end to a contentious subplot in the search for dark matter, the mysterious stuff whose gravity binds the galaxies. Today, researchers working with the Large Underground Xenon (LUX) detector at the Sanford Underground Research Facility in Lead, South Dakota, announced that they see no signs of the lightweight dark matter particles hinted at by other experiments. "It's a significant miss," says Richard Gaitskell, a LUX team member from Brown University. "We should have seen thousands of events and we simply don't see any."

LUX stalks weakly interacting massive particles (WIMPs), physicists' best guess at what makes up dark matter. In addition to producing gravity, WIMPs would interact with other matter and themselves only through the weak nuclear force. And if the newborn universe swarmed with particles that can interact through only that force, then just enough of them would remain to supply the dark matter, so long as they weighed between one and 1000 times as much as a proton. WIMPs also typically emerge from a concept called supersymmetry, which posits a more massive partner for every particle in physicists’ prevailing model and suggests that WIMPs should be a few hundred times as heavy as a proton.

In recent years, however, a few groups have reported signs of unexpectedly light WIMPs weighing less than 10 times as much as a proton. In 2010, physicists working with the Coherent Germanium Neutrino Technology (CoGeNT) experiment reported possible signs of lightweight WIMPs striking their 440-gram detector in the Soudan mine in northern Minnesota. However, physicists with the ongoing XENON experiment in Italy's subterranean Gran Sasso National Laboratory argued that their data ruled out light WIMPs. A squabble then ensued over whether XENON researchers properly calibrated their detector, which was filled with 100 kilograms of frigid liquid xenon.

By most accounts, the most credible hint of light WIMPs came in April. Physicists working with the Cryogenic Dark Matter Search (CDMS) in Soudan reported three clean events that were consistent with light WIMPs crashing into the ultracold disks of silicon in their detector.

Now, however, LUX researchers say their results refute the CDMS signal and the other hints of light WIMPs. Lurking 1480 meters underground, LUX cost $10 million and started taking data in April. It contains 370 kilograms of liquid xenon, making it more sensitive than either XENON or CDMS. Had CDMS detected WIMPs and not extraneous background radiation, LUX researchers should have seen roughly 1600 events during the 85 days they took data, Gaitskell says. They saw none.

So will the LUX results settle the debate over light WIMPs? The key is how well LUX researchers have calibrated their detector and whether they can show that it's truly sensitive to low-mass WIMPs, says Juan Collar, a physicist at the University of Chicago in Illinois and leader of the CoGeNT team. "It will all depend on what [energy] threshold they achieve and how cautious they have been" in their analysis, Collar says.

Of course, the real goal is to detect WIMPs, light or heavy. LUX researchers will take data for another 2 years. They then plan to build a 7-tonne detector called LZ. Meanwhile, XENON researchers plan to start up a 1-tonne version of their experiment next year, and CDMS researchers are upgrading their detector, too.

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