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Constance Walter

Last week, the Large Underground Xenon (LUX) collaboration announced a whole new level of sensitivity for its dark matter experiment. Although no dark matter particles were found, LUX's sensitivity far exceeded the goals for the project. The results give researchers confidence that if a particle had interacted with the detector's xenon target, they almost certainly would have seen it. 

"It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone," said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for LUX. 

The new results allow scientists to eliminate many potential models for dark matter particles, offering critical guidance for the next generation of dark matter experiments. The final results were announced at the Identification of Dark Matter 2016 conference and signaled the completion of a 300-live-day search that ended in May. 

During a 20-month run, the LUX team incorporated unique calibration measures to search a wide swath of potential parameter space for dark matter particles called WIMPs, or weakly interacting massive particles.

"These careful background-reduction techniques and precision calibrations and modeling, enabled us to probe dark matter candidates that would produce signals of only a few events per century in a kilogram of xenon," said Aaron Manalaysay, the Analysis Working Group coordinator for LUX and a research scientist from UC Davis, who presented the new results in Sheffield, UK.  

With the completion of its final run, LUX is preparing for decommissioning this fall. But before that, the LUX team plans to use the detector to continue calibrating and testing backgrounds in preparation for the next generation dark matter detector, LUX-ZEPLIN (LZ). 

"The main driver behind this campaign of calibrations is to test new techniques or improve on existing techniques, which will be used for LZ," said Simon Fiorucci, a physicist at Lawrence Berkeley National Laboratory and science coordination manager for the experiment. LUX has sufficient size, low-enough background and a known response that can tell researchers if the techniques will work.

Fiorucci said some interesting science also can come out of some of these tests. For example, the neutron generator studies done in June and July could further improve understanding of the xenon response to WIMP interactions at extremely low energy. "This would be a boon to LZ, LUX and the entire field of dark matter," he said.

The LZ team also plans to measure the intrinsic radioactivity of a liquid scintillator mix that will be used with LZ and requires an extremely quiet environment. The scintillator will replace LUX inside the high-purity water tank. 

"This critical piece of information will tell LZ whether their background is good enough for the outer detector to perform as expected and, if not, where they should focus their efforts to make it so," Fiorucci said. 

The tests will run through January.