Scientists at an ambitious dark matter experiment in operation at an underground gold mine in South Dakota have discovered exactly what they thought they would: nothing. And the results have got them really excited.
After running the detector for three months and looking for tiny flashes of light that could indicate a dark matter particle collision, researchers with the Large Underground Xenon experiment at the Homestake mine have found no signals beyond the expected background noise. But they did so at far better sensitivities than any such experiment before them, the researchers said.
It's "the best dark matter detector result out there right now.… Everyone will be talking about it," LUX scientist Richard Gaitskell of Brown University said in a presentation at the Sanford Underground Research Facility, where the detector is located.
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The results from the experiment deepen the mystery around dark matter, the strange stuff that makes up about 85% of the matter in the universe but can't be seen or touched. But its presence is felt throughout the universe, in the motions of galaxies and the cosmic web thought to connect galaxy clusters.
Scientists want to find out what this stuff is, but it's made up of stuff so weak – probably something scientists call weakly interacting massive particles, or WIMPs – that they've never been able to detect it.
LUX is the latest in a line of dark-matter experiments that's sought to capture the exceedingly faint signals from the extremely rare occasions when a dark matter particle may interact with normal matter – the stuff that makes up the universe we know.
The scientists ran the experiment for 85 days, using the clean center of a tank of ultra-purified xenon, which itself sits in a 70,000-gallon tank of water to protect it from radiation.
They found no sign of dark matter signals, Gaitskell said. Their ultra-sensitive detector, which picks up flashes of light given off when a particle knocks into a xenon atom, picked up 1.9 events each day – 160 total – but they could pretty much all be attributed to background noise, the researchers said.
The physicists say this is essentially what they had expected. In February, when the detector was first turned on, Brown University physicist Simon Fiorucci said it was unlikely they would find dark matter any time soon.
"We will have not found dark matter better than everyone else," he said at the time.
Now, the detector's high sensitivity appeared to surprise the researchers.
Scientists don't know how heavy a WIMP is, and their detector can look for a wide range of particle sizes, measured in energy. Their detector was about twice as sensitive to heavier particles as the previous experiment and 20 times more sensitive in the lighter end of the mass scale, Gaitskell said.
The researchers are nowhere near done. After this initial 85-day run, they're planning a yearlong campaign that could even further improve their readings.
As for whether that will happen soon, the jury is out. One thing is clear: The researchers seem a lot more optimistic.
In February, Fiorucci didn't think a dark matter detection was around the corner. Now, his answer is different.
"Discovery, or at least the first hint of a discovery at the next phase, is not entirely ruled out; I think that's the fair thing to say," he said in an interview.
So could LUX pick up on a hint of a dark matter signal in the near future?