The LUX-ZEPLIN (LZ) experiment team has announced the results from its first scientific run today; the experiment is the world’s most sensitive dark matter detector, and though it did not find any dark matter in this first round, the team confirmed that the experiment is working as expected.
The LZ experiment detector is made up nested tanks of liquid xenon, each 1.5 metres tall and 1.5 metres wide, buried under North Dakota. The idea is that a dark matter particle whizzing through space will eventually bounce off one of the xenon atoms, knocking loose electrons in a flash that is recorded by the experiment. The tank is buried about a mile below Earth’s surface to minimise the amount of background noise. Today’s announcement comes after 60 live days of data collection that spanned December 25 to May 12.
“We’re looking for very, very low-energy recoils by the standards of particle physics. It’s a very, very rare process, if it’s visible at all,” said Hugh Lippincott, a physicist at UC Santa Barbara and a member of the LZ team, in a press conference today. “You can shoot a dark matter particle through 10 million light-years of lead and expect only one interaction at the end of that light-year.”
Dark matter is the catch-all term for the unknown stuff that appears to make up about 27% of the universe. It hardly ever interacts with ordinary matter, hence its “darkness” to us. But we know it’s out there because, while never directly detected, it has gravitational effects that can be seen on cosmic scales. (NASA breaks down the concept pretty well here.)
There are many candidates for dark matter. One is the WIMP, or a Weakly Interacting Massive Particle. Unlike other dark matter hypotheses like axions or dark photons, which are so small and diffuse that they may behave more like waves, WIMPs would have mass but hardly ever interact with ordinary matter. So to detect them, you need a device that pretty much mutes all other physics going on.
LZ is super sensitive, which makes it good for spotting such fleeting and infrequent interactions. The experiment is 30 times larger and 100 times more sensitive than its predecessor, the Large Underground Xenon experiment, according to a Sanford Underground Research Facility release. LZ is “effectively an onion” Lippincott said, with each layer of the experiment insulating against noise that could obscure a potential WIMP interaction.
“The collaboration worked well together to calibrate and to understand the detector response,” said Aaron Manalaysay, a physicist at Berkeley Laboratory and a member of the LZ team, in a Berkeley Lab press release. “Considering we just turned it on a few months ago and during COVID restrictions, it is impressive we have such significant results already.”
Of the many detections the LZ experiment made in its 60-day run, 335 seemed promising, but none turned out to be WIMPs. That doesn’t mean WIMPs aren’t out there, but it does eliminate a mass range from contention. (This is the crux of what dark matter detectors do: bit by bit, they rule out what masses the particles can’t be.) Multiple physicists recently told Gizmodo that they think the next big discovery in particle physics will come from a dark matter detector like LZ.
This scientific run kicked off what’s expected to be a 1,000-day schedule. The recent round was also unblinded, so the LZ team could monitor how the technology behaved. Since it performed as expected, the next scientific run will have its results ‘salted,’ or peppered with phony signals, to mitigate bias.
Twenty times more data will be collected in the coming years, so perhaps the wimpy WIMPs will finally have to face the music of their own existence. Then again, maybe they don’t exist at all. We won’t know until we look.