Around eighty-five per cent of the matter scientists have detected in the universe comes from something we can’t feel or see. It’s a seemingly enormous amount of mass whose gravity bends other stars’ light and makes galaxies spin strangely. And scientists really, really want to know what this so-called dark matter is.
But how do you detect something you can’t see or feel? If dark matter is a tiny particle, as many theories predict, then the solution is giant vats of liquid xenon, an element that’s usually a gas at room temperature, buried deep in mine shafts or in mountains. And the biggest functioning vat, an experiment called XENON1T buried beneath a mountain in Gran Sasso, Italy has just released its first results. There’s still no sign of dark matter—but no one’s losing hope yet.
“I think the most exciting thing is the fact that the detector works as we expect,” Laura Baudis, professor at the Physik Institut of the University of Zurich told Gizmodo.
But why vats of liquid xenon? Right now, physicists have compelling reasons to believe that dark matter should be some sort of particle that only interacts very weakly with the nucleus of regular-matter atoms. Physicists are hoping that those particles will hit the liquid xenon nuclei, producing light particles or knocking off an electron. The time between the initial photon signal from the strike and another photon signal from a released electron migrating out of the experiment determines where in the chamber the dark matter would have struck. Photomultiplier tubes amplify the signal and show up as a blip above some background on a graph.
Folks at XENON1T aren’t too worried about the lack of a detection just yet—their results, published today on the arXiv physics preprint server, were only based on about a month’s worth of data. If you put a big bowl in your backyard and waited for a meteor to hit it, you wouldn’t say “meteors don’t exist” just because you hadn’t caught one in a month. Especially if, in the case of dark matter, the meteors pass right through the bowl and the only way to detect that you’ve caught one is through a faint blip of light it might leave on a camera.
That’s essentially what these kinds physics experiments do. Once scientists have proven that there’s no dark matter at the mass detectable by the experiment’s operating sensitivity (that usually takes a few years) they move onto more sensitive (read: bigger) detectors. Bigger experiments increase the odds of actually detecting something, and that means more xenon.
“Every time we run our detector longer or make it bigger, we’re exploring more of the parameter space,” Christopher Tunnell, fellow at the Kavli Institute for Cosmological Physics at the University of Chicago, told Gizmodo. “You’re able to say dark matter isn’t this or it isn’t this.”
You’re probably wondering, if these detectors are so sensitive, how do they know they’ve spotted dark matter and not something else? University of California, San Diego’s XENON1T principle investigator Kaixuan Ni explained to me that radiation can come from anywhere and cause a signal in the detector, so XENON1T is buried deep underground to keep out stray particles from space. The scientists also learn what naturally-occurring atoms of radioactive elements might look like in the detector, so they can cut any of those signals out during data analysis. XENON1T is also shielded by water, and its newest results only include data from the middle of the detector, using the outer layers of XENON as additional shielding.
This has long been the way physicists have slowly been ruling out the possible properties that dark matter particles have had. XENON1T stands for XENON 1 tonne, because it contains a tonne (well, actually a little more than three tonnes) of liquid xenon. It used to be called XENON100, and before that XENON10. Competing detectors are looking for dark matter particles in similar ways—the Large Underground Xenon experiment (LUX) finished its dark matter search without a particle to show for it last summer, and is currently upgrading to “LZ.” Then there’s PandaX (again, a vat of xenon) and others that use another noble gas, argon. These noble gasses are used because they release light and electrons when they’re smacked, according to an article in symmetry magazine.
Folks from the LUX/LZ experiment and others outside the physics community have been paying close attention to the competition. XENON is first out of the gate of the newest iteration of these experiments. “This is the next generation coming out of its childhood in some sense,” Bob Jacobsen, physicist at the University of California, Berkeley, who works on LUX and LZ, told Gizmodo. “They’re not just showing that photomultiplier tubes are working, but actually doing physics.” And while not speaking on behalf of LZ, Jacobsen said the pressure is certainly on, now. “Everyone is focused on getting the next experiment built. It’s hard to win against someone whose detector is 3-4 times bigger than yours.”
Others thought the new results weren’t a huge leap, yet. Kathryn Zurek, theoretical physicist at Lawrence Berkeley National Laboratory in California, told me that XENON1T’s results only barely pushed past last year’s LUX results, which ruled out dark matter particles within a certain mass range. She did point out that these weakly-interacting dark matter detectors are now in “production mode,” chugging along looking for hints of particles.
But, as with the “competing” ATLAS and CMS experiments at the Large Hadron Collider that jointly discovered the Higgs boson, it’s important to have some independent verification in the case of a discovery. “We need two experiments,” said Ni. “If XENON1T discovers dark matter signals, then LZ can confirm it.”
As these experiments are getting larger, folks are beginning to feel the pressure of what might happen should we fail to discover dark matter. “You can’t do this forever,” said Tunnell. “You wonder if maybe dark matter is different from what you expect it to be.” In other words, not a weakly-interacting particle. Scientists aren’t at that level yet, said Baudis, and are working towards the ultimate dark matter detector, called DARWIN. But once these experiments get sensitive enough that tiny particles emanating from the sun and outer space called “neutrinos” start to turn up as hits in the detector, then it might be time to throw in the towel. “If we haven’t seen any dark matter [by] then, then it would be too many neutrinos,” she said. It’s not a hard cutoff, said Zurek, but it would take a whole lot more xenon to hunt for a slight dark matter particle interaction in the neutrino sea.
“So the question is going to become after LZ is built: Are we going to build another generation of experiments?” asked Zurek. “Now we’re talking about quantities of xenon getting to be a nontrivial fraction of the world’s supply.”
In that case, scientists would need to hunt for dark matter in different ways. This is something scientists are already discussing, said Zurek.
But we’re not there just yet—the current xenon vats are hunting in “that sweet spot” where weakly-interacting dark matter might communicate with our experiments via particles we know about and can detect. So for now, the hunt is on. Said Baudis: “We simply have no way of knowing until we look.” [arXiv]