The LUX-ZEPLIN (LZ) experiment team today announced the results of its first scientific run; The experiment is the world’s most sensitive dark matter detector, and while it didn’t find any dark matter in this first round, the team confirmed that the experiment is working as expected.
The LZ experiment’s detector consists of nested tanks of liquid xenon, each 1.5 meters high and 1.5 meters wide, buried under South Dakota. The idea is that a particle of dark matter hurtling through space would eventually bounce off one of the xenon atoms, knocking off electrons in a flash recorded by the experiment. The tank is buried about a mile underground to minimize the amount of background noise. Today’s announcement comes after 60 days of live data collection from December 25th to May 12th.
“We are looking for very, very low-energy recoils according to the standards of particle physics. It’s a very, very rare process, if it’s visible at all,” Hugh Lippincott, a physicist at UC Santa Barbara and a member of the LZ team, said in a news 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 that light-year.”
Dark matter is the collective term for the unknown material that makes 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 it has gravitational effects visible on cosmic scales, although it’s never been directly detected. (NASA breaks down the concept pretty well here.)
There are many dark matter candidates. One is the WIMP, or a weakly interacting massive particle. in contrast to others Hypotheses about dark matter like axions or dark photons, which are so small and diffuse that they might behave more like waves, WIMPs would have mass but would hardly ever interact with ordinary matter. So to detect them, you need a device that pretty much mutes all other physical processes.
LZ is very sensitive, which makes it good at 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 Sanford Underground Research Facility publication. LZ is “effectively an onion,” Lippincott said, with each layer of the experiment insulating against noise that might obscure a potential WIMP interaction.
“The collaboration worked well to calibrate and understand the detector response,” said Aaron Manalaysay, a physicist at the Berkeley Laboratory and a member of the LZ team, at a Berkeley Lab press release. “Considering we only turned it on a few months ago and during the COVID lockdown, it’s impressive that we already have such significant results.”
Of the many discoveries the LZ experiment made during its 60-day run, 335 seemed promising, but none turned out to be WIMPs. That doesn’t mean there are no WIMPs, but it eliminates a mass area from the competition. (That’s the crux of what dark matter detectors do: bit by bit, they rule out what the masses of the particles are tip be.) Several physicists recently said to Gizmodo that they think the next big discovery in particle physics will come from a dark matter detector like LZ.
This scientific run started what is expected to be a 1,000-day schedule. The final lap was also unblinded, allowing the LZ team to monitor how the technology was behaving. Since it worked as expected, the results of the next scientific run will be “salted” or peppered with false signals mitigate prejudices.
Twenty times more data will be collected in the coming years, so perhaps the weak WIMPs will finally have to face the music of their own existence. On the other hand, they may not even exist. We won’t know until we look.
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