One of the fundamental challenges of researching dark matter is our inability to detect it. While it constitutes an estimated 27% of all the estimated mass and energy in the observable universe, it doesn’t interact with any type of electromagnetic radiation. Scientists have worked for years to try and find direct evidence of dark matter’s existence, but to little avail. After its last, 20-month run, the Large Underground Xenon (LUX) dark matter experiment team reported that they had failed to detect any of the particles they were looking for.
“LUX has delivered the world’s best search sensitivity since its first run in 2013,” said Rick Gaitskell, physics professor at Brown University and co-spokesperson for the LUX project. “With this final result from the 2014-2016 search, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is 4 times better than the original project goals. 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.”
The LUX detector is one of a kind, an exquisitely sensitive detector designed to pick up signs of weakly interacting massive particles (WIMPs) during one of the rare interactions between dark matter and normal matter. LUX consists of a third of a ton of cooled liquid xenon in a titanium shell, surrounded by powerful sensors designed to detect the photons and electrical charges emitted if a WIMP should manage to collide with a xenon atom. The whole shebang is shielded from cosmic rays by a 72,000-gallon tank of high-purity water and a mile of rock; it’s located in the former Homestake Gold Mine in Lead, South Dakota. It ended up being an even better detector than anticipated, partly because the team had to work so hard to isolate the baseline of impostor particle interactions that their baseline error filtration got really good. But, since they got so good at picking out the baseline, they can say with certainty that the baseline is all they observed. They did not see dark matter interactions.
In their statement, the LUX collaboration pointed out that while they had eliminated “large swathes” of possible mass ranges and interactions associated with WIMPs, the WIMP model itself “remains alive and viable.” Constraints don’t invalidate the model. Data you didn’t anticipate is still data.
Now it’s a matter of time: The next data on dark matter might come from CERN, or it might come from LUX’s successor, LUX-ZEPLIN, which will be 70 times as sensitive, taking LUX’s place in the mine. Compared with LUX’s one-third-ton of liquid xenon, LZ will have a 10-ton liquid xenon target, which will fit inside the same 72,000-gallon tank of pure water used by LUX to help fend off external radiation.
“The innovations of the LUX experiment form the foundation for the LZ experiment,” said Harry Nelson of UCSB, spokesperson for that project. “We expect LZ to achieve 70 times the sensitivity of LUX. The LZ program continues to pass its milestones, aided by the terrific support of the Sanford Lab, the DOE, and its many collaborating institutions and scientists. LZ should be online in 2020.”