A super-sensitive dark matter detector has just started

A super-sensitive dark matter detector has just started

The central LZ detector in the clean room of the Sanford Underground Research Facility.

The central LZ detector in the clean room of the Sanford Underground Research Facility.
Photo: Matthew Kapust, Sanford Underground Research Facility

The LUX-ZEPLIN (LZ) experiment team announced today the results of its first scientific campaign; the experiment is the most sensitive dark matter detector in the world, and although it found no dark matter in this first round, the team confirmed that the experiment is working as expected.

The LZ experiment’s detector is made up of nested tanks of liquid xenon, each 1.5 meters high and 1.5 meters wide, buried under the Dakota. The idea is that a dark matter particle whizzing through space will eventually bounce off one of the xenon atoms, dropping electrons in a flash recorded by the experiment. 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 which took place from December 25 to May 12.

“We are 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,” Hugh Lippincott, a physicist at UC Santa Barbara and a member of the LZ team, said at a press conference today. “You can shoot a dark matter particle through 10 million light-years of lead and expect a single interaction at the end of that light-year.”

Dark matter is the catch-all term for the unknown matter that appears to make up about 27% of the universe. It almost never interacts with ordinary matter, hence its “darkness” to us. But we know it’s there because, although it’s never directly detected, it has gravitational effects that can be observed on a cosmic scale. (NASA breaks down the concept pretty well here.)

There are many candidates for dark matter. One is the WIMP, or Weakly Interacting Massive Particle. unlike others hypotheses of dark matter like axions or dark photons, which are so small and diffuse that they can behave more like waves, WIMPs would have mass but almost never interact with ordinary matter. So to detect them you need a device that pretty much cuts out all other physics going on.

LZ is super sensitive, making it ideal for spotting those fleeting, 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 mask a potential WIMP interaction.

The LZ outdoor detector, which protects against unwanted signals.

The LZ outdoor detector, which protects against unwanted signals.
Photo: Matthew Kapust, Sanford Underground Research Facility

“The collaboration worked well to calibrate and understand the detector response,” said Aaron Manalaysay, a Berkeley lab physicist and member of the LZ team, in a Berkeley lab. Press release. “Given that we just activated it a few months ago and during the COVID restrictions, it’s impressive that we already have such significant results.”

Of the many detections made by the LZ experiment during its 60 days, 335 looked promising, but none turned out to be WIMPs. That doesn’t mean the WIMPs aren’t there, but it does eliminate a mass range of contention. (That’s the heart of what dark matter detectors do: bit by bit, they remove what’s massing the particles can not be.) Several 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 race launched what should be a 1,000-day calendar. The recent cycle was also opened, so the LZ team was able to monitor the behavior of the technology. Since it worked as expected, the next scientific campaign will see its results “salty”, or sprinkled with false signals, for reduce bias.

Twenty times more data will be collected in the years to come, so perhaps the 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.

Plus: 10 years after the Higgs boson, what’s the next big thing for physics?

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