The search for dark matter is underway underground. In the first LUX-ZEPLIN (LZ) experiment, which lasted more than three months and ended this April, the world’s most sensitive dark matter detector found no sign of hypothetical dark matter particles called weakly interacting massive particles (WIMPs) in the cosmos. Despite the lack of hard data, scientists have confirmed that the US Department of Energy-led experiment is working as planned, leaving open the possibility of finding dark matter in future rounds.
“For now it’s kind of weird, we say we’re the best in the world at not discovering anything, but the prospect of finding new physics in a few years is quite feasible,” Chamkaur Gag, astroparticle physicist and professor at University College in London and a member of the LUX experiment team, said A new scientist.
Dark matter is estimated to make up 27 percent of the universe (the visible matter in stars and galaxies may only make up 5 percent of it). That said, no one ever discovered it. This is because dark matter contains particles that do not emit, absorb or reflect light, making it difficult to even measure with electromagnetic radiation. But physicists and astronomers know that dark matter exists because of the gravitational effects it has on visible objects, such as keeping stars from flying into space and keeping galaxies from collapsing. It is supposed to be the invisible glue that holds the universe together.
[Related: Meet the mysterious particle that’s the dark horse in dark matter]
WIMPs are scientists’ best bet for detecting dark matter. Other hypothetical dark matter such as photons or axions are very small and behave like waves. But WIMPs contain mass and rarely interact with other visible matter. Billions of WIMPs also pass through us every second. By studying dark matter, experts will better understand what the true basis of the universe is and what we can expect to happen to it in the future.
The key to unlocking the secrets of the universe is buried a mile beneath the Black Hills of South Dakota. The LZ experiment consists of two subsequent titanium tanks filled with 10-ton tanks of pure liquid xenon. It also contains two arrays of photomultiplier tubes that can detect even the faintest light. If dark matter in the form of WIMPs collides with a xenon atom, it will knock off the free electrons. The collision of particles causes a brief flash of brightness that the LZ experiment captures.
The experiment is underground because cosmic radiation and radiation from human bodies can drown out dark matter signals. So submerging the detector helps increase its sensitivity and the chances of finding a sign of dark matter. “You’re trying to hear a whisper. If you do it in the middle of New York, you won’t hear it, there’s just too much noise. You want to escape our background — the cosmic rays and junk we’re bombarded with would hide the very rare signals we’re looking for,” Kevin Lesko, a senior physicist at Lawrence Berkeley National Laboratory who coordinates the LZ project, said. Popular science in 2020
[Related: What we learned from the Large Hadron Collider on its first day back in business]
Although the first round of results did not detect dark matter, it showed that the machine was performing well and functioning as expected. “Given that we only turned it on a few months ago and during the COVID restrictions, it’s impressive that we already have such significant results,” said Aaron Manalaisey, the physics coordinator at the Berkeley lab who is leading the effort to the initial run of the experiment, in a Berkeley Lab press release.
“We’re now out of the starting gate,” Harry Nelson, a UC Santa Barbara physics professor and former LZ spokesman, said in a second press release. “LZ is a much more powerful dark matter detector than any that has been built so far and is uniquely capable of making a discovery in the next few years.”