The elusive particle: Scientists embark on a quest to find dark matter
Scientists are pretty sure invisible objects make up most of the mass of the universe and say we wouldn’t be here without them – but they don’t know what they are. The race to solve this great mystery has taken one team to the depths beneath Lead, South Dakota.
The question for scientists is basic, says Kevin Lesko, a physicist at Lawrence Berkeley National Laboratory. “What is this great 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 out nearly all of the cosmic rays and particles that glide around – and penetrate – all of us every day. But dark matter particles, scientists say, can avoid all those obstacles. They hoped someone would fly into a vat of liquid xenon in the inner tank and hit the xenon like two balls in a game of billiards, revealing its existence in a flash of light seen by a device called a “time projection space.”
Scientists announced Thursday that the five-year, $60 million search was finally launched two months ago after delays caused by the COVID-19 pandemic. So far the device has found … nothing. At least no dark matter.
It’s all right, they said. The equipment seems to work to filter out most of the background radiation they want to block. “To look for this very rare type of interaction, job number one is to first get rid of all the usual sources of radiation, which would overwhelm the experiment,” said University of Maryland physicist Carter Hall.
And if all of their calculations and theories were correct, they thought they would see only a few signs of dark matter a year. The team of 250 scientists predicts they will get 20 times more data over the next few years.
By the time the experiment is complete, the chances of finding dark matter with this device “may be less than 50% but more than 10%,” Hugh Lippincott, physicist and spokesman for the experiment, told a press conference Thursday.
While that’s far from certain, “you need a little enthusiasm,” says Lawrence Berkeley’s Lesko. “You don’t go into rare quest physics without the hope of finding something.”
Two giant Depression-era hoists run the elevator that takes scientists to the so-called LUX-ZEPLIN experiment at the Sanford Underground Research Facility. The 10-minute drop ends in a tunnel with cool walls lined with nets. But a stuffy old mine soon leads to a high-tech lab where dirt and contamination are the enemy. The helmet was swapped for a cleaner one and a double layer of baby blue boots covered the steel-toed safety boots.
At the heart of the experiment is a 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 cool and minimize background radiation.
Xenon is special, explains experimental physics coordinator Aaron Manalaysay, because it allows researchers to see whether a collision occurred with one of its electrons or with its nucleus. If something hits the nucleus, it’s most likely the dark matter everyone is looking for, he said.
These scientists tried a similar, smaller experiment here many years ago. After it came empty, they thought they should go much bigger. Another large-scale experiment is underway in Italy run by a rival team, but so far no results have been announced.
Scientists are trying to understand why the universe is not as it seems.
One part of that mystery is dark matter, which by far has the most mass in the universe. Astronomers knew it was there because when they measured the stars and other regular matter in the galaxy, they found that there wasn’t enough gravity to hold the cluster together. If nothing else was out there, the galaxy would “quickly fly apart,” Manalaysay said.
“It is fundamentally impossible to understand our observations of the historical, 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 a lot of doubt about what it is. The main theory is that it involves things called WIMPs – weakly interacting massive particles.
If that’s the case, LUX-ZEPLIN can detect it. We wanted to find “where the cowards can hide,” Lippincott said.
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