Scientists from South Dakota to Australia seek material believed to make up quarter of universe as researchers in China investigate another mystery particle
What could be bigger than gravitational waves? Predicted by Einstein, confirmed to exist this week, they are born of black holes colliding and the sound of space time itself warping through the Earth.
What couldn’t be bigger, say scientists still pining for answers to the other mysteries of physics.
The discovery of dark matter, argued cosmologist Carlos Frenk at the annual meeting of the American Association for the Advancement of Science, would be more important than the detection of gravitational waves.
The latest results for the most sensitive dark matter detector in the world came in in December, and physicist Alex Murphy delivered them to the Guardian on Saturday. His team’s experiment is run nearly a mile down a former gold mine in South Dakota, with a vat of liquid xenon.
But Murphy said his team could not upstage anyone yet – they have still not detected the mysterious material that is thought to make up a quarter of the universe.
The experiment was recently upgraded: cooling a vat containing a third of a ton of xenon to -150F, it means to detect the rare event of a xenon atom colliding with a dark matter particle, creating a tiny flash.
This has not yet happened, Murphy said.
“But it’s certainly good news that more regions have been ruled out,” he said, referring to the various places scientists have been searching for dark matter. “There’s oodles and oodles of expected models,” he said. “It’s entirely conceivable that we find a signal very, very soon. But it’s also conceivable it’s going to take a long time.”
Physicists believe some kind of dark matter exists because astronomers can detect the gravitational pull it exerts on stars and galaxies. But dark matter itself is not only invisible, it has never been detected directly or indirectly by other means. Scientists have suggested that dark matter comprises “weakly interacting massive particles” (Wimps), leftover material from the big bang that pass through normal matter and leave no trace but in gravity.
“It’s a bigger quest than just trying to find it,” said Murphy, of the University of Edinburgh. “Once we see it that’s not the end of the road – it’s the key to unlocking what the deeper theory of physics is, and beyond the shadow of a doubt that’s going to be a long quest.”
The South Dakota scientists, whose lab is called the Large Underground Xenon (LUX), are not the only hunters at large. In Australia, a team is building the world’s latest dark matter detector under another gold mine (the caverns help shield the instruments from cosmic rays that could create a false alarm). In outer space, a detector is mounted on the International Space Station in the hopes that it can find an indirect sign of dark matter, like a rafter trying to make sense of ripples forming in water. Results are also expected from other vat experiments and particle colliders in the next year.
“It does make it quite competitive,” Murphy said, noting that LUX data had excluded what had seemed like tantalizing hints from other labs. “But if one of the competitors comes up with a very clear, distinctive signal for dark matter I think we’ll all be very happy.”
Over the next few years LUX will get another overhaul and about 10 tons of liquid xenon, essentially becoming a new detector named LUX_Zeplin, Murphy said, but the technical challenges mean results will likely have to wait until 2018.
“You’ve got to keep control of 10 tons of liquid xenon a mile underground, knowing that you can’t afford to lose it because it’s very expensive, and that much cryogenic gas could be potentially be very dangerous. So you have a zero failure requirement.”
At AAAS researchers looking for another type of mysterious particle, a fourth type of neutrino, called “sterile” because it does not carry even the weak charges as does its neutral counterparts – the normal neutrinos. Kam-Biu Luk, a physicist at the University of California at Berkeley, announced new data showing an “unexpected disagreement between our observation and predictions” in the search for the particles.
He and his colleagues at an experiment in China, the Daya Bay Collaboration, found an extraordinary excess of antineutrinos there, in line with two other experiments. They published their results on Saturday in the journal Physical Review Letters.
Cosmologist Olga Mena Requejo suggested that should the new neutrinos be found, scientists could learn more about the relationship between matter and antimatter, and answer a question hovering over physics since the 1950s: “What is the neutrino character? Because it’s something unique, and it’s really, really important.”
And even for the scientists who do find what they’ve been seeking after decades of searching, life goes on. “In a certain way we’re still going through an existential crisis,” said Tim Andeen, one of the hundreds of scientists who helped find the Higgs boson particle exactly as they’d hoped to in 2012, at the Large Hadron Collider in Geneva.
“We had a thing to go and search for, and we got it,” he said. “Things would’ve gotten really weird if we hadn’t – we would’ve observed all kinds of things in the detector.”
Andeen, now at the University of Texas at Austin, said the work for new discoveries continue, though now researchers have a wider field to search for: signatures of supersymmetry, extra dimensions, dark matter. “We don’t have a Higgs boson to look for anymore, but we do know the Higgs boson can’t be the end of the story.”
Last year scientists at the collider found a tantalizing blip in the data, for instance, that Murphy said would be “incredibly hard to reconcile with the standard model”. But Andeen quickly cautioned that so far, at least, the signal is “statistically speaking not important”, and that it could easily go away as other bumps have before.
“We may or may not have something exciting to say in the next year.”