In a former gold mine a mile underground, inside a titanium tank filled with a rarefied liquefied gas, scientists have begun the search for what has so far been undetectable: dark matter. Scientists are pretty sure that invisible stuff makes up most of the mass of the universe, and they say we wouldn’t be here without it, but they don’t know what it is. The race to solve this massive mystery has led one team to the depths beneath Leyde, South Dakota.
The question for scientists is fundamental, says Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory. “What is this great place I live in? Right now, 95% of it is a mystery.” The idea is that a mile of dirt and rock, a giant tank, a second tank, and the world’s purest titanium will block almost all of the cosmic rays and particles that move around—and through—all of us every day. But dark matter particles, scientists believe, can avoid all these obstacles. They hope that one will fly into the container of liquid xenon in the inner tank and crash into the xenon core like two balls in a pool game, revealing its existence in a flash of light seen by a device called a “projection camera the time”. Scientists announced Thursday that the five-year, $60 million search finally began two months ago after a delay caused by the COVID-19 pandemic.
So far, the device hasn’t detected… anything. At least there is no dark matter. That’s OK, they say. The equipment appears to work to filter out most of the background radiation they hoped to block. “To look for this very rare kind of interaction, the number one task is to first get rid of all the ordinary sources of radiation that would complicate the experiment,” said
And if all their calculations and theories are correct, they think they’ll only see a few fleeting signs of dark matter a year. The team of 250 scientists estimates that they will get 20 times more data in the next few years. By the time the experiment is finished, the chance of finding dark matter with this device is “probably less than 50%, but more than 10%,” Hugh Lippincott, a physicist and spokesman for the experiment, said at a press conference Thursday. While it’s far from a sure thing, “you need some enthusiasm,” said Lawrence Berkeley’s Lesko.
“You don’t go into physics looking for rare objects without some hope of finding something.” Two massive Depression-era hoists operate an elevator that takes scientists to the so-called LUX-ZEPLIN experiment at the Sanford Underground Research Center. A 10-minute descent ends in a tunnel with cool-to-the-touch mesh-lined walls. But the musty old mine soon leads to a high-tech laboratory where dirt and pollution are the enemy. Helmets are replaced with new cleaner ones and double layer baby blue boots go over steel toed safety boots. The heart of the experiment is a giant tank called a cryostat, lead engineer Jeff Cherwinka said on a tour in December 2019 before the device was released closed and full.
He describes it as “like a thermos” made of “perhaps the purest titanium in the world” designed to keep liquid xenon cold and keep background radiation to a minimum. Xenon is special, explained experiment physics coordinator Aaron Manalaisey, because it allows researchers to see whether the collision is with one of its electrons or its nucleus. If something hits the core, it’s more likely to be the dark matter everyone’s looking for, he said. These scientists tried a similar, smaller experiment here years ago.
After coming up empty, they decided they needed to go much bigger. Another large-scale experiment is underway in Italy, led by a rival team, but no results have been announced so far. Scientists are trying to understand why the universe is not what it seems. One part of the mystery is dark matter, which has most of the mass in the cosmos. Astronomers know it’s there because when they measure the stars and other ordinary matter in galaxies, they find that there isn’t enough gravity to hold those clumps together.
If nothing else, the galaxies would “disperse rapidly,” Manalaisei said. “It is essentially impossible to understand our observational history, of the evolutionary cosmos, without dark matter,” Manalaisey said. Lippincott, a physicist at the University of California, Santa Barbara, said “we wouldn’t be here without dark matter.” So while there is no doubt that dark matter exists, there are many doubts about what it is. The leading theory is that it involves things called WIMPs – Weakly Interacting Massive Particles. If so, LUX-ZEPLIN can detect them. We want to find “where the weak points might be hiding,” Lippincott said.