Located deep beneath the Black Hills of South Dakota, the world’s largest dark matter detector has begun operations to hunt for this elusive and mysterious form of matter.
The LUX-ZEPLIN (LZ) experiment is especially attuned to detect a particular type of hypothetical dark matter particle, called Weakly Interacting Massive Particles, or WIMPs. The detector first began operations in December 2021, and its first results are in. The findings, published on the LZ experiment website, prove it to be the most sensitive dark matter probe in the world—sensitive enough, its operators hope, to directly detect dark matter for the very first time. In the process, the 250 or so scientists who operate LZ could solve one of science’s most pressing questions.
The dark matter puzzle is so important to physicists because the ordinary baryonic matter that comprises the everyday “stuff” we see around us—as well as all of the universe’s planets, stars, and gas clouds—makes up only 5 percent of its matter and energy budget. The rest represents the so-called “dark universe” made up of mysterious dark energy and dark matter.
While dark energy, which powers the accelerating expansion of the universe, accounts for 70 percent of this budget, dark matter makes up the remaining 25 percent. This means when energy is taken out of the equation, dark matter accounts for over 80 percent of the universe’s contents.
“We live in a special time, most of the universe is a mystery that we can not currently explain. What is dark matter? What is dark energy?” Kevin Lesko, Berkeley Lab senior physicist and a past LZ spokesperson, tells Popular Mechanics. “Physicists, cosmologists, and astronomers are working very diligently right now to better understand these.”
What Are WIMPs?
Detecting dark matter is so tough despite its abundance because it doesn’t interact with electromagnetic radiation. This is what tells us that dark matter is not made up of atoms comprised of protons and neutrons (baryons ), and is very different from ordinary matter.
The only way astronomers have been able to infer its presence is via its interaction with gravity. If it wasn’t for the gravitational effects of dark matter, galaxies are spinning so fast that they would fly apart. But, there could be another way to detect dark matter. If it is understood with ordinary matter of WIMPs, it should interact very weakly baryonic matter, also known as visible matter.
WIMPs are believed to be heavy and slow-moving particles with masses between one and 1,000 times that of a proton, created in the early universe as other subatomic particles collided. As the universe expanded and cooled, these collisions became increasingly rare. Dark matter theories suggest that this process left us with exactly enough WIMPs to account for the amount of dark matter we measure today, about five times as much as ordinary matter.
The interaction of WIMPs with baryonic matter occurs through the weak nuclear force — one of the universe’s four fundamental forces — and this so slight it requires an immensely sensitive detector to spot it. That’s where the LZ experiment comes in.
What Is the LUX-ZEPLIN Experiment?
The LZ experiment contains a multilevel detection system and exists in environmental conditions precisely designed to eliminate as much “noise” from background sources as possible. Planning for the detector began nine years ago with its fabrication at the Sanford Laboratory in South Dakota beginning two years later.
At the heart of the LZ detector are two titanium vats containing around ten tonnes of liquid xenon. This xenon must be maintained at -148 degrees Fahrenheit (-100 degrees Celsius); scientists selected this element for the experiment because it can be made extremely dense and very pure. As the interaction between WIMPs and ordinary matter is proposed to be very rare, the more xenon the detector can use, the better the chance of catching such an event. The LZ experiment’s xenon “target” is larger than that of any other dark matter detector.
The xenon vats are ringed by a photomultiplier tube (PMT) array containing 500 sensitive light detectors designed to spot faint flashes of light. “When particles collide with xenon atoms, the xenon atoms scintillate — giving off a small flash of light, and the atomic electrons can be knocked off as the xenon is ionized,” Lesko explains. “The electrons are drifted [to] the top of the xenon by applying an electric field to it. When the electrons reach the top, they are pulled out of the liquid by a stronger electric field and into a layer of gas on top of the liquid. Once there, the electrons produce a second flash of light.”
The difference in time between these two flashes allows the researchers to determine the depth in the tanks at which the interaction occurred. “Therefore we can determine the 3D position of each event,” Lesko says. “The two flashes of light give us information about the energy of the interaction and help us distinguish background events from potential WIMP events.”
Around this setup is a water tank that helps protect the experiment from background radiation. “We controlled the radioactivity of every component in the detector by assaying every item, every nut, bolt, screw, wire, piece of Teflon, and piece of tin in the instrument, producing the most radio-pure dark matter detector ever,” Lesko says. “The careful engineering, fabrication, and integration of the components, which came from around the world, create a very finely tuned and effective instrument for looking for dark matter interactions and rejecting background radiation.”
Outside of this is a second, larger detection system. The purpose of this is to eliminate false positives by detecting particles that could create dark matter-like interactions, particularly neutrons. The LZ experiment is also protected from high-energy charged particles from the sun and other cosmic sources — called cosmic rays — by its location nearly a mile underground.
The Results So Far
While the first results from the LZ experiment did not contain any indications of dark matter, Lesko points to several positive aspects of the initial research that was collected over the first three months of its operations.
“I have three major take-away points from this first analysis. Firstly, our backgrounds are as low as we expected. Secondly, the detector is operating well and is able to effectively look for dark matter,” Lesko says. “Finally, it is working so well, that with three months of data we have been able to establish a world-leading result, surpassing the earlier experiments, many of which operated for years.”
🤯 More Mind-Bending Physics
Over the next 1,000 days, the sensitivity of the LZ experiment will be further boosted, meaning its operators are right to be optimistic about its potential contribution to our understanding of the universe.
“We are anxiously looking forward to what we will find in the coming years,” Lesko concludes. “We are hopeful that entirely new chapters will be written for our textbooks, media articles, and popular press in the near future.
“This is an exciting time for us all.”
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