The LZ central detector in the clean room of the Sanford underground research facility. Photo: Matthew Kapust, Sanford Underground Research Facility
The LUX-ZEPLIN (LZ) experimentation team announced today the results of its first scientific test; the experiment is the most sensitive dark matter detector in the world, and while it found no dark matter in that first round, the team confirmed that the experiment works as expected.
The LZ experiment detector consists of liquid xenon nest tanks, each 1.5 meters high and 1.5 meters wide, buried under South Dakota. The idea is that a particle of dark matter flowing through space will eventually bounce off one of the xenon atoms, hitting loose electrons in a flash that is recorded by the experiment. The tank is buried 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 spanning from December 25 to May 12.
“We’re looking for a very, very low energy setback by the standards of particle physics. It’s a very, very rare process, if it’s visible,” said Hugh Lippincott, a physicist at UC Santa Barbara and a member of the LZ, at a press conference today. “You can shoot a particle of dark matter through 10 million light-years of lead and expect only one interaction at the end of this light-year.”
Dark matter is the general term for unknown things that seem to account for about 27% of the universe. It almost never interacts with ordinary matter, hence its “darkness” for us. But we know it is there because, although it has never been detected directly, 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 WIMP, or a massive particle with weak interaction. Unlike other dark matter hypotheses such as axions or dark photons, which are so small and fuzzy that they can behave more like waves, WIMPs would have too much but would almost never interact with ordinary matter. Therefore, to detect them, you need a device that silences virtually all other physics.
LZ is super sensitive, which makes it good for detecting 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 statement from the Sanford Underground Research Facility. LZ is “effectively an onion,” Lippincott said, with each layer of the experiment isolating the noise that could obscure a possible WIMP interaction.
The LZ external detector, which protects against unwanted signals. Photo: Matthew Kapust, Sanford Underground Research Facility
“The collaboration worked well to calibrate and understand the detector’s response,” Aaron Manalaysay, a Berkeley Laboratory physicist and LZ team member, said in a Berkeley Lab press release. “Given that we launched it a few months ago and during COVID restrictions, it’s impressive that we already have such significant results.”
Of the many detections the LZ experiment made during its 60 days, 335 looked promising, but none turned out to be WIMP. This is not to say that WIMPs are not there, but it does eliminate a massive range of the dispute. (This is the crux of what dark matter detectors do: little by little, they rule out what masses particles cannot have.) Several physicists recently told Gizmodo that they believe the next great discovery in particle physics will come from a dark matter. detector with LZ.
This scientific career started what is expected to be a 1,000-day program. The recent round was also shown to be blind, so the LZ team was able to control how the technology behaved. Because it worked as expected, the next scientific test will have its results “salty” or salted with false signals, to mitigate the bias.
Twenty times more data will be collected in the coming years, so perhaps WIMPs will have to finally face the music of their own existence. Then, they may not exist at all. We won’t know until we look.
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