Dark Matter: The Quest for the Invisible Begins in an Old Gold Mine | Science

In a former gold mine, a mile underground, in a titanium tank filled with a rare liquefied gas, scientists have begun searching for the previously untraceable: 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, said 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 on Thursday that the five-year, $60 million search has finally begun after a delay two months ago caused by the pandemic. So far, the device has found … nothing. At least not dark matter.

An all-white chamber, viewed from a low angle, is lined with dome-shaped devices, all aimed at a clear cylinder.
Scientists are hoping that a particle of dark matter will fly into a container of liquid xenon and smash into a nucleus to prove its existence. Photo: Matthew Kapust/AP

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 Thursday.

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

A laboratory worker wears a full body protective suit with a head and face covering in a room covered with plastic sheeting.
Laboratory workers are careful not to contaminate the dark matter detector at the Sanford Underground Research Facility in Lead, South Dakota. Photo: Stephen Groves/AP

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.

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

These scientists tried a similar, smaller experiment here years ago. Having come up empty handed, they figured they had to go much bigger. Another large-scale experiment is underway in Italy, led by a competing team, but no results have been announced so far.

Scientists are trying to understand why the universe is not what it seems.

A team of scientists in full-body hazmat suits stand in a row near a device that appears to be made up of concentric cylinders, an outer transparent and an inner opaque, stretching from floor to ceiling.
The research team stands next to the giant tank called the cryostat, which one scientist compared to a thermos flask. Photo: Matthew Kapust/AP

Part of the puzzle is dark matter, which is by far the most massive in the cosmos. Astronomers know it’s there because when they measure the stars and other normal matter in galaxies, they find that gravity isn’t nearly enough to hold these clusters together. If nothing else was out there, galaxies would “fly apart quickly,” Manalaysay said.

“It’s basically impossible to understand our observation of history, of the evolutionary cosmos 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 little doubt that dark matter exists, there is much doubt as to what it is. The leading theory is that these are things called wimps – weakly interacting massive particles.

If that’s the case, Lux-Zeplin might spot them. We want to find out “where the wimps can hide,” Lippincott said.

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