August 14, 2022

Dark Matter: The Search for the Unseen Begins in an Ancient Gold Mine Science

In a former gold mine one mile underground, inside a titanium tank filled with a rare liquefied gas, scientists began searching for what had hitherto been undetectable: dark matter.

Scientists are pretty sure that invisible things make up most of the mass of the universe and say we wouldn’t be here without them – but they don’t know what it is. The race to solve this formidable mystery sends one team into the depths under the lead of bullets, South Dakota.

The question for scientists is fundamental, said Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory. “What is this wonderful 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 all the cosmic rays and particles that orbit around and through us almost every day. But scientists believe that dark matter particles can avoid all those obstacles. They hope one will fly into the liquid xenon bowl in the inner tank and collide with a xenon core like two billiard balls, revealing its presence in a flash of light seen by a device called a “time-projection chamber”.

Scientists announced Thursday that the five-year, $60 million research process finally began two months ago after a delay caused by the pandemic. So far the device hasn’t found anything. At least there is no dark matter.

An all-white chamber, seen from a low angle, lined with dome-shaped hardware, all aimed at a clear cylinder.
Scientists hope that a dark matter particle will fly into a container of liquid xenon and collide with a nucleus to prove its existence. Photo: Matthew Capost/Associated Press

They say it’s okay. The device appears to filter out most of the background radiation they hope to block. “To search for this very rare type of interaction, the first task is to first get rid of all the normal sources of radiation, which would overwhelm the experiment,” said Carter Hall, a physicist at the University of Maryland.

And if all their calculations and theories are correct, they imagine that they will see only a few passing signs of dark matter each year. The team of 250 scientists estimates that they will have 20 times more data over the next two years.

By the time the experiment ends, the chance of finding dark matter with this device is “probably less than 50% but more than 10%,” Hugh Lippincott, a physicist and spokesperson for the experiment said at a press conference Thursday.

While that’s far from certain, “you need a little enthusiasm,” Lesko said. “Don’t get into rare search physics without the hope of finding something.”

A laboratory worker wears a full protective suit with head and face covering in a room covered with a plastic sheeting.
Laboratory workers are careful to avoid contaminating the dark matter detector at the Sanford Underground Research Facility in Lead, South Dakota. Photo: Stephen Groves/The Associated Press

Two huge Depression-era cranes operate an elevator that brings scientists to the so-called Lux ​​Zeppelin Experiment at the Sanford Underground Research Facility. The 10-minute descent ends in a tunnel with cold walls lined with a net. But the old, rotting mine soon leads to a high-tech laboratory where dirt and pollution are the enemy. The helmets are replaced with new, cleaner ones, and the double layer of blue infant socks are replaced with steel-toed safety shoes.

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

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

These scientists tried a similar, smaller experiment here years ago. After they came out empty, they thought they had to go a lot bigger. Another large-scale trial is taking place in Italy run by a rival team, but no results have been announced yet.

Scientists are trying to understand why the universe appears to be so different.

A team of scientists wearing full-body protective suits stands in a row near a device that appears to be concentric cylinders, with a transparent exterior and an opaque interior, extending from floor to ceiling.
The research team stands next to a giant tank called a cryostat, which one scientist likened to Thermos. Photo: Matthew Capost/Associated Press

One piece of the puzzle is dark matter, which has by far the most mass in the universe. Astronomers know it exists because when they measure stars and other ordinary matter in galaxies, they find that there is almost not enough gravitational pull to hold these clusters together. If nothing else, Manalaisay said, the galaxies would “be flying quickly.”

“It is impossible to understand our observation of history, of the evolutionary universe without dark matter,” Manalisay said.

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

So while there is no doubt about the existence of dark matter, there is a lot of doubt about what it is. The leading theory is that it includes things called Wimps – that interact weakly with massive particles.

If so, Lux-Zeplin may be able to detect it. We want to find “where cowards can hide,” Lippincott said.