A vast subterranean search for mysterious dark matter begins

LEAD, SD — In a former gold mine, a mile underground, in a titanium tank filled with a rare liquefied gas, scientists have begun searching for what hasn’t been found before: dark matter.

Scientists are pretty sure the invisible stuff makes up most of the mass of the universe and say we wouldn’t be here without it – but they don’t know what it is. The race to solve this enormous mystery has taken a team led by Lead, South Dakota, deep.

The question for scientists is fundamental, says Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory. “What is this great place that I live in? Right now, 95% of that is a mystery.”

The idea is that a mile of dirt and rock, a giant tank, a second tank, and the world’s purest titan will block almost all of the cosmic rays and particles that penetrate and surround us every day. But dark matter particles, scientists believe, can circumvent all of these obstacles. They hope one will fly into the vat of liquid xenon in the inner tank and smash into a xenon core like two balls in a game of billiards, revealing its existence in a flash of light seen by a device called the “Time Projection Chamber”.

Scientists announced Thursday that the five-year, $60 million search has finally begun after a delay two months ago caused by the COVID-19 pandemic. So far, the device has found … nothing. At least not dark matter.

That’s fine, 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 task is to first eliminate any ordinary sources of radiation that would overwhelm the experiment,” said University of Maryland physicist Carter Hall.

And if all their calculations and theories are correct, they expect to see only a few glimpses of dark matter per year. The team of 250 scientists estimate that they will receive 20 times more data in the next few years.

By the end of the experiment, the probability of finding dark matter with this device is “probably below 50% but above 10%,” Hugh Lippincott, physicist and spokesman for the experiment, said at a news conference on Thursday.

That’s far from certain, but “you need a little enthusiasm,” said Lawrence Berkeley’s Lesko. “You don’t go into rare search physics without any hope of finding something.”

Two giant Depression-era hoists operate an elevator that takes scientists to the so-called LUX-ZEPLIN experiment at the Sanford Underground Research Facility. A 10-minute descent ends in a tunnel with cool-to-the-touch walls lined with mesh. But the musty old mine soon leads to a high-tech laboratory where dirt and contamination are the enemy. Helmets are swapped out for new cleaner ones and a double layer of baby blue ankle boots is pulled over steel-toed safety boots.

At the heart of the experiment is the giant tank called the cryostat, lead engineer Jeff Cherwinka said on a tour in December 2019, before the device was closed and filled. He described it as “like a thermos” made of “perhaps the purest titanium in the world” designed to keep the liquid xenon cold and background radiation to a minimum.