It's official: Scientists with the Large Underground Xenon dark-matter detector have combed through 20 months of pristine data and found, buried deep in the measurements ... nothing out of the ordinary.
The findings, presented at the Identification of Dark Matter conference in Sheffield, England, were not unexpected — though they do highlight the challenge of finding the elusive stuff known as dark matter.
"I couldn't say with a straight face that I was expecting to find dark matter with this particular data set," said Simon Fiorucci, an experimental physicist at Lawrence Berkeley National Laboratory and science coordination manager for LUX. "We were hoping for a good surprise, but we are not counting on good surprises."
Dark matter can't be seen, heard or felt – but scientists know something must be there because they watch how its enormous mass turbocharges the spin of galaxies. Everything we can detect in the universe, from Earth to the stars, black holes and distant galaxies – all of it makes up less than 5% of the mass and energy in the universe. Dark matter makes up nearly 27% (outnumbering normal matter by more than 5 to 1), and yet it has remained one of the most intractable mysteries of the cosmos for decades.
Scientists don't know what dark matter is, but many think it might be some kind of particle. To that end, they've set up a range of experiments to capture direct evidence of dark matter. The LUX detector lies nearly a mile underground in a former South Dakota gold mine. The device consists of one-third of a ton of ultra-pure liquid xenon, which sits stately and silent, waiting for the exceedingly rare event when a standoffish dark matter particle might – just might – make contact with one of its atoms.
LUX came online in 2013; the campaign in this analysis, from October 2014 to May 2016, yielded its most sensitive analysis yet, six times better than the 2013 results. And yet, the researchers' thorough examination of the data found nothing other than background noise.
"I can't say I was surprised. Disappointed, yes, because we are still, despite appearances to the contrary, in the business of trying to find something — but I don't think many people were very surprised," Fiorucci said. However, he added, "it's according to expectations, which as far as we're concerned is already a good result."
At base, the LUX campaign showed that the technology was sound, and performed even better than previously expected, Fiorucci said. That is good news, since LUX will be taken apart in its underground cavern this fall and winter to make way for the upgraded version, LZ (short for LUX-ZEPLIN).
With 10 tons of liquid xenon (compared with the mere third of a ton in LUX), LZ is expected to be about 70 times as sensitive as LUX. Though the current campaign couldn't find anything, LZ might have a better chance.
"One has to put this in some sort of historical perspective," said Brown University astrophysicist Richard Gaitskell, a spokesperson for the experiment. "I've been looking for dark matter for some 28 years now, and we started using detectors that weighed tens of grams."
In short, they've made quite a bit of progress in the last few decades, the scientists said.
Another silver lining: The fact that LUX did not find anything might help to narrow down some of the models for dark matter, Gaitskell said.
Of course, there's always the possibility that this and similar detectors are on the wrong track, because dark matter isn't some kind of weakly interacting particle at all.
But Fiorucci said that such questions will have to wait for about a decade, until these extremely sensitive detectors can be put to the test.
"We are limited by the technology at the moment and not really by the theory," he said.
And just as giant telescopes built for one purpose have discovered many unexpected objects along the way, LUX's successors may be able to pick up other phenomena on the side, Gaitskell added, such as a long-predicted but as-yet undetected effect known as the coherent scattering of solar neutrinos.
"Nobody had assumed LUX would be able to do this," Gaitskell said, "but we got a lot closer to it than people had imagined was possible."
In fact, the scientists will keep analyzing the data for the next year and a half or so, on the hunt for other more exotic processes that theorists have suggested might exist, he added.
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July 22, 6:05 p.m.: The story was updated with comments from an interview with Richard Gaitskell.