Fiat LUX: Dark-matter detector’s initial results

The nature of the dark matter that makes up some 85% of the material stuff of the universe remains a mystery. Current consensus favors a weakly interacting massive particle, or WIMP, inspired by extensions of the standard model of particle physics. One approach to understanding those putative particles is to look for their rare interactions with conventional matter. A number of experiments have undertaken such searches. Last week the most sensitive of them, the LUX (Large Underground Xenon) experiment, announced via webcast its first results. The experiment’s initial three-month run saw no evidence of WIMPs, which significantly lowered the upper limits for the mass-dependent WIMP interaction strength. Moreover, the new results ruled out the handful of possible light-WIMP sightings that had been reported earlier. Located 1.5 km belowground at the Sanford Underground Research Facility in Lead, South Dakota, the LUX detector monitors 250 kg of liquid xenon. When a WIMP collides with a xenon atom, the recoiling atom plows through the scintillating liquid, generating photons received by photomultiplier tubes (such as those shown in the figure) at the top and bottom of the detector. Details of the photomultiplier signals reveal the energy and position of the WIMP interaction. The experiment is difficult in part because cosmic-ray particles and electrons from radiogenic decay can trigger a WIMP-like signal. The detector’s underground location mitigates the cosmic-ray problem; a surrounding tank filled with pure water provides additional shielding. The LUX researchers plan to collect data for another two years. The collaboration’s next-generation detector, already being planned, will contain 20 times the xenon of LUX and will be so sensitive that it will see background neutrinos. (D. S. Akerib et al., LUX collaboration, http://arxiv.org/abs/1310.8214 .)