Deep inside an old gold mine, in a vat of liquid xenon, a new hunt has begun for dark matter — the mysterious stuff that makes up about 85% of all matter in the universe.
Nobody knows what exactly dark matter is. Scientists know it exists because they can measure the way its gravity affects faraway galaxies, but they’ve never detected it directly. That’s the goal of a new experiment buried deep beneath Lead, South Dakota: to catch dark matter in the act of interacting with other particles.
The experiment is called LUX-ZEPLIN, or LZ for short. It’s a 10-ton vat of pure liquid xenon, rigged with detectors to catch the incredibly faint flash of energy that would come from a particle of dark matter colliding with a xenon atom. Researchers announced Thursday that it is online and ready to search for new particles.
“Dark matter remains one of the biggest mysteries of particle physics today,” Hugh Lippincott, a spokesperson for the LZ team of 250 scientists, said in the livestreamed announcement.
After eight years of preparation, the LZ detector worked as expected during a 63-day test run, according to the researchers, who published a report on that first set of data on Thursday. Now they’re preparing to run the experiment for up to 1,000 days, starting in late summer or early fall. They could have early results sometime in 2023, but the observation could continue for up to five years.
This isn’t the first vat of liquid xenon to search for dark matter, but it’s the largest and most sensitive. Its new data has ruled out a range of masses for dark-matter particles, and it has enough sensitivity to search in even lower mass ranges.
If they do uncover a new particle, it could lead to a new, more precise physics beyond the Standard Model that has defined our understanding of the universe since the 1970s. The detection of dark matter would revolutionize our most fundamental understanding of the universe.
“Everyone’s trying to find some some evidence for physics beyond the standard model. And probably the strongest evidence we have of that is dark matter,” Aaron Manalaysay, the project’s physics coordinator, told Insider, adding, “But we just really don’t know what it is.”
To spot dark matter, you have to set a very quiet stage
Dark matter could come from Weakly Interacting Massive Particles (WIMPs) — a theoretical particle that would interact with gravity and occasionally, very rarely, collide with particles of visible matter. That’s a leading theory, but nobody has detected a WIMP before. That’s the main thing the LZ project is looking for.
You could shoot a WIMP through 10 million light-years of lead and only get one collision, Lippincott said.
Luckily, if they exist, many WIMPs should be passing through us all the time. In 10 tons of xenon atoms, there should be regular collisions. The experiment just needs to be quiet enough that the faint, fleeting signal of the WIMPs isn’t lost to background noise.
“Our job is to get some chunk of matter, which is very clean and very quiet from a particle perspective, and in which we can instrument and be able to detect when there was a particle interaction,” Manalaysay said.
That’s why the researchers built the LZ detector in the Sanford Underground Research Facility, an old gold mine nearly a mile underground. The location protects it from background noise. For further quiet, the xenon is nested inside two titanium tanks.
The LZ search for dark matter is a process of elimination. Most of its sensors are designed to identify signals that match a known particle interaction — something that is definitely not dark matter.
“That’s really the name of the game here in the dark-matter-search field, is having a large detector and having a very low rate of background signals,” Manalaysay said.
A global search for invisible matter
Because it has the largest tank of liquid xenon yet, and because of its quiet location, LZ is the most sensitive dark-matter detector on Earth. It’s not the only one, but it will be the most sensitive to potential WIMPs.
In China, a 4-ton xenon experiment called PandaX published its first results in December.
A similar experiment in Italy, called XENON1T, announced in 2020 that it had detected an unexpectedly high number of collisions in its latest run. None of them look like dark matter, but they could point to a different new particle. Data from the test run of the LZ detector should shine some light on what those collisions might be, Manalaysay said.
XENON1T, the LZ team, and a large group of dark-matter scientists in Europe, called DARWIN, have formed an enormous consortium of hundreds of scientists. Eventually, they plan to build a giant dark-matter experiment together — “one more xenon experiment to rule them all,” Lippincott said — though there is currently no timeline for that project.