Dark matter: the search for the invisible begins in an old gold mine | Science

In an old gold mine a kilometer underground, inside a titanium tank filled with a rare liquefied gas, scientists began the search for what until now had not been found: dark matter .

Scientists are pretty sure the invisible stuff 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 massive mystery has taken a team deep beneath Lead, South Dakota.

The question for scientists is fundamental, said Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory. “What is this wonderful place where I live? 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 purest titanium in the world 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 avoid all of these obstacles. They hope one will fly into the vat of liquid xenon in the inner tank and slam 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 chamber. temporal”.

Scientists announced Thursday that the five-year, $60 million search finally began two months ago after a delay caused by the pandemic. So far, the device has found…nothing. At least no dark matter.

An all-white bedroom, seen from a low angle, is lined with dome-shaped devices, all directed towards a clear cylinder.
Scientists hope a dark matter particle will fly into a vat of liquid xenon and crash into a nucleus to prove its existence. Photography: Matthieu Kapust/AP

It’s OK, they say. The equipment seems to work to filter out most of the background radiation they hoped to block. “To search for this very rare type of interaction, task number one is to get rid of all ordinary sources of radiation, which would overwhelm the experiment,” said University of Maryland physicist Carter Hall.

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

At the end of the experiment, the chances of finding dark matter with this device are “probably less than 50% but greater than 10%”, said Hugh Lippincott, physicist and spokesman for the experiment during a press conference on Thursday.

While it’s far from a sure thing, “you need a little bit of enthusiasm,” Lesko said. “You don’t go into rare research physics with no hope of finding something.”

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

Two huge Depression-era winches operate an elevator that takes scientists to what is called the Lux-Zeplin experiment in the underground Sanford Research Facility. A 10-minute descent ends in a tunnel with cool-to-the-touch walls lined with netting. But the moldy old mine soon leads to a high-tech laboratory where dirt and contamination are the enemy. Helmets are swapped for new, cleaner ones, and a double layer of sky blue booties go over steel-toed safety boots.

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

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

These scientists tried a similar, smaller experiment here years ago. After arriving empty they thought they had to go a lot 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 figure out why the universe is not what it seems.

A team of scientists wearing full protective suits stand in line near a device that appears to be concentric cylinders, a transparent exterior and an opaque interior, which spans from floor to ceiling.
The research team stands next to the giant tank called the cryostat, which one scientist has compared to a thermos. Photography: Matthieu Kapust/AP

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 stars and other regular matter in galaxies, they find that there isn’t enough gravity to hold these clumps together. If there was nothing else out there, the galaxies would “rapidly separate,” Manalaysay said.

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

Lippincott, a physicist from 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 are many doubts that it is. The main theory is that it involves things called Wimps – weakly interacting massive particles.

If so, Lux-Zeplin might be able to detect them. We want to find “where the wimps can hide,” Lippincott said.

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