Scientists are pretty sure invisible matter 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 brought a team into the depths under South Dakota’s Lead. The question for scientists is a key one, said Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory. “What is this wonderful place I live in? Right now, 95% is a mystery.” The idea is that a mile of dirt and rock, a giant tank, a second tank, and the purest titanium in the world will block almost all of the cosmic rays and particles that pass around and through us all every day. But dark matter particles, scientists believe, can avoid all these obstacles. They hope that someone will fly into the reservoir of liquid xenon in the inner tank and smash a xenon core like two balls in a pool game, revealing its existence in a flash of light seen from a device called a “time projection chamber.” Scientists announced Thursday that the five-year, $60 million quest finally began two months ago after a delay caused by the pandemic. So far the device has found … nothing. At least not dark matter. Scientists hope that a dark matter particle will fly into a tank of liquid xenon and crash into a nucleus to prove its existence. Photo: Matthew Kapust/AP It’s okay, they say. The equipment appears to be working to filter out most of the background radiation they were hoping to block. “To look for this very rare type of interaction, the first job is to first get rid of all the usual sources of radiation, which would overwhelm the experiment,” said physicist Carter Hall of the University of Maryland. And if all their calculations and theories are correct, they estimate that they will only see a few fleeting signs of dark matter per year. The team of 250 scientists estimates they will receive 20 times more data over the next two years. By the time the experiment is over, the chance of finding dark matter with this device is “probably less than 50% but more than 10%,” Hugh Lippincott, a physicist and experiment spokesman, said at a news conference Thursday. While that’s far from a sure thing, “you need some excitement,” Lesko said. “You don’t engage in the rare physical search without some hope of finding something.” Lab workers take care to avoid contaminating the dark matter detector at the Sanford Underground Research Center in Lead, South Dakota. Photo: Stephen Groves/AP Two massive Depression-era elevators operate an elevator that brings scientists to what’s called the Lux-Zeplin experiment at the underground Sanford Research Center. A 10-minute descent ends in a tunnel with cool net-lined walls. But the old, musty mine soon leads to a high-tech laboratory where dirt and contamination are the enemy. The helmets are exchanged for new cleaner ones and a double layer of blue boots over the steel toed safety boots. The heart of the experiment is the giant tank called the cryostat, chief engineer Jeff Cherwinka said on a tour in December 2019 before the device was shut down and filled. He described it as “like a thermos” made of “perhaps the purest titanium in the world” designed to keep liquid xenon cold and keep ambient radiation to a minimum. Xenon is special, experiment physics coordinator Aaron Manalaysay explained, because it allows researchers to see whether a 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 realized they had to go much bigger. Another large-scale experiment is underway in Italy run by a rival group, but so far no results have been announced. Scientists are trying to understand why the universe is not what it seems. The research team stands next to the giant tank called a cryostat, which one scientist likened to a Thermos. Photo: Matthew Kapust/AP Part of the mystery is dark matter, which has by far the most mass in the universe. Astronomers know they are there because when they count the stars and other normal matter in galaxies, they find that there is not nearly enough gravity to hold these clusters together. If there was nothing else out there, the galaxies would “fly away fast,” Manalaysay said. “It is virtually impossible to understand the observational history of the evolutionary world without dark matter,” Manalaysay 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 is much doubt about what it is. The leading theory is that it involves things called Wimps – weakly interacting massive particles. If this is the case, Lux-Zeplin could detect them. We want to find “where the dudes can hide,” Lippincott said.
