Massive Underground Dark Matter Detector Comes Up Empty Handed

Brett Smith for redOrbit.com – Your Universe Online

Just three months into its operation the Large Underground Xenon (LUX) experiment is already the most sensitive dark matter detector in the world, according to scientists from Brown University.

“LUX is blazing the path to illuminating the nature of dark matter,” said Rick Gaitskell, professor of physics at Brown University and project spokesperson.

The detector sits over a mile below the Sanford Underground Research Facility in South Dakota. Deep in the Earth, the device is better able to detect the uncommon, weak interactions between dark matter particles and ordinary matter, according to project researchers. The results of the project’s initial 90-day run were announced during a seminar on Wednesday at the Sanford Lab in Lead, SD.

“What we’ve done in these first three months of operation is look at how well the detector is performing, and we’re extremely pleased with what we’re seeing,” Gaitskell said. “This first run demonstrates a sensitivity that is better than any previous experiment looking to detect dark matter particles directly.”

The leading theoretical candidates for dark matter particles are called weakly interacting massive particles (WIMPs). The LUX scientists said WIMPs could be either in a high-mass or low-mass form.

The results from the initial run suggest LUX has twice the sensitivity of any other dark matter detection experiment to detect high-mass WIMPs. The new results indicate potential detections of low-mass WIMPS by other experiments were probably the result of background radiation, not dark matter, the researchers said.

“There have been a number of dark matter experiments over the last few years that have strongly supported the idea that they’re seeing events in the lowest energy bins of their detectors that could be consistent with the discovery of dark matter,” Gaitskell explained. “With the LUX, we have worked very hard to calibrate the performance of the detector in these lowest energy bins, and we’re not seeing any evidence of dark matter particles there.”

The LUX researchers are set to begin a 300-day run that could either definitively find dark matter or rule out a vast area of  space where it could have existed.

“Every day that we run a detector like this we are probing new models of dark matter,” Gaitskell said. “That is extremely important because we don’t yet understand the universe well enough to know which of the models is actually the correct one. LUX is helping to pin that down.”

To capture the incredibly rare interactions that would signify the presence of dark matter, the LUX workers have engineered a highly-sensitive detector. The most important part of the LUX is a third of a ton of supercooled xenon in a tank outfitted with light sensors. These sensors are capable of detecting a single photon at a time. When a particle interrelates with the xenon, it creates a miniscule flash of light and an ion charge, both of which can be seen by the sensors.

In order to minimize background interactions, the detector had to be protected from background radiation and cosmic rays, which is why the LUX sits nearly 4,900 feet underground, in 71,600 gallons of pure de-ionized water.

“LUX is a huge step forward. Within the first few minutes of switching it on, we surpassed the sensitivity of the first dark matter detectors I was involved with 25 years ago,” Gaitskell said. “Within a few days, it surpassed the sensitivity of sum total of all previous dark matter direct search experiments I have ever worked on. This first LUX run is more sensitive than any previous search conducted and now sets us up perfectly for the 300-day run to follow.”

“We are very excited that our thesis work has culminated in this world-leading result,” said Jeremy Chapman, a graduate student and LUX researcher.