A huge underground search for mysterious dark matter begins

LEAD, SD (AP) — In an old gold mine a mile below ground, inside a titanium tank filled with a rare liquefied gas, scientists have begun a search for what has so far been impossible to find: matter dark.

Scientists are pretty sure that invisible matter makes up most of the mass in 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 to the depths of Lead, South Dakota.

The question for scientists is a basic one, says Kevin Lesko, a physicist at the Lawrence Berkeley National Laboratory. “What is this great 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 world’s purest titanium will block almost all of the cosmic rays and particles that surround and pass through us every day. But dark matter particles, scientists believe, can bypass all those obstacles. They expect one to fly into the vat of liquid xenon in the internal tank and crash into a xenon core like two balls in a game of pool, revealing its existence in a flash of light seen by a device called “the projection camera of the weather”.

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 has found…nothing. At least no dark matter.

It’s okay, they say. The team appears to be working to filter out most of the background radiation they hoped to block out. “To look for this very rare type of interaction, job number one is to get rid of all ordinary sources of radiation first, which would overwhelm the experiment,” said physicist Carter Hall of the University of Maryland.

And if all their calculations and theories are correct, they think they will only see a couple of fleeting dark matter signals a year. The team of 250 scientists estimates that they will obtain 20 times more data in the next two years.

By the time the experiment is finished, the chance of finding dark matter with this device is “probably less than 50% but greater than 10%,” Hugh Lippincott, a physicist and spokesman for the experiment, said at a news conference on Thursday.

While that’s far from a sure thing, “it takes a little bit of enthusiasm,” said Lawrence Berkeley’s Lesko. “You don’t go into weird looking physics without some hope of finding something.”

Two giant Depression-era forklifts drive an elevator that takes scientists to what’s called the LUX-ZEPLIN experiment at the Sanford Underground Research Center. A 10-minute descent ends in a tunnel with cool-to-the-touch walls lined with netting. But the musty old mine soon leads to a high-tech lab where dirt and pollution are the enemy. Hard hats are swapped out for new, cleaner ones and a double layer of baby blue ankle boots is layered over steel-toed safety boots.

The heart of the experiment is the giant tank called a cryostat, lead engineer Jeff Cherwinka said on a December 2019 tour 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 background radiation to a minimum.

Xenon is special, explained experimental physics coordinator Aaron Manalaysay, because it allows researchers to see if a collision is with one of its electrons or with its nucleus. If something hits the core, it’s more likely to be the dark matter everyone’s looking for, he said.

These scientists attempted a similar, smaller experiment here years ago. After running out, they figured they had to go much bigger. Another large-scale experiment is underway in Italy run 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 by far most of the mass in the cosmos. Astronomers know it’s there because when they measure the stars and other regular matter in galaxies, they find that there isn’t enough gravity to hold these clusters together. If there were nothing else out there, the galaxies would be “quickly blowing to bits,” Manalaysay said.

“It is essentially impossible to understand our observation of history, of the evolving cosmos without dark matter,” said Manalaysay.

Lippincott, a physicist at the University of California, Santa Barbara, said “we wouldn’t be here without dark matter.”

So while there is little 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 that is the case, LUX-ZEPLIN could detect them. We want to find “where the weak can hide,” Lippincott said.


Follow Seth Borenstein on Twitter at @borenbears.


The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.

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