New Super-Sensitive Dark Matter Detector Booted

The LUX-ZEPLIN (LZ) experimental team has announced the results of its first scientific test today; the experiment is the world’s most sensitive dark matter detector, and although it didn’t find any dark matter in this first round, the team confirmed that the experiment worked as expected.

The LZ experimental detector consists of nested liquid xenon tanks, each 1.5 meters high and 1.5 meters wide, buried beneath North Dakota. The idea is that a dark matter particle streaking through space will eventually bounce off one of the xenon atoms, knocking the electron loose in the instant the experiment recorded. The tank is buried about a mile below the earth’s surface to minimize the amount of background noise. Today’s announcement comes after 60 days of live data collection that ran from December 25 to May 12.

“We are looking for very, very low energy recoil by particle physics standards. This is a very, very rare process, if seen at all,” Hugh Lippincott, a physicist at UC Santa Barbara and a member of the LZ team, said at a press conference today. “You can shoot dark matter particles through 10 million light-years of lead and expect only one interaction at the end of that light-year.”

Dark matter is an umbrella term for the unknown that seems to make up about 27% of the universe. It almost never interacts with ordinary matter, therefore “darkness” for us. But we know it’s out there because, while never directly detected, it has gravitational effects that can be seen on a cosmic scale. (NASA breaks down the concept pretty well here.)

There are many candidates for dark matter. One of them is WIMP, or Weakly Interacting Massive Particles. Unlike other hypothesized dark matter such as 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 it, you need a device that pretty much silences all the other physics going on.

LZ is very sensitive, which makes it great to see such brief 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 release from the Sanford Underground Research Facility. LZ is “effectively an onion” Lippincott said, with each layer of the experiment isolating noise that could obscure potential WIMP interactions.

LZ Outdoor Detector, which guards against unwanted signals. (Photo: Matthew Kapust, Sanford Underground Research Facility)

“This collaboration worked well together to calibrate and 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. “Given we just activated it a few months ago and during the COVID restrictions, it’s impressive we’ve had significant results already.”

Of the many detections the LZ experiment performed in 60 days, 335 seemed promising, but none turned out to be WIMP. That’s not to say WIMP isn’t out there, but it does take mass ranges out of contention. (This is the essence of what dark matter detectors do: little by little, they override the mass of the particles can not b.) Several physicists recently told Gizmodo that they think the next big discovery in particle physics will come from dark matter detectors like LZ.

This scientific run kicks off what is expected to be a 1,000-day schedule. The recent round is also not covered, so the LZ team can monitor how the technology behaves. As it performs as expected, subsequent scientific research will have the results ‘salted’, or peppered with false signals, to reduce bias.

Twenty times as much data will be collected in the coming years, so perhaps weaker WIMPs will eventually have to face the music of their own existence. Then again, maybe they don’t exist at all. We won’t know until we see.

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