Popular Science



- By Ryan Bradley

Our latest shot at spying dark matter—and thus revealing the makeup of the universe—sits nearly a mile undergroun­d

WE WERE PACKED IN TIGHT: a crew of some 30 physicists, engineers, biologists, and—mostly—miners. Ex-miners, actually. This hadn’t been an active mine for 18 years. The guy working the lift let the winch operator above us know that the cab was full, that we were a go. For a brief, delirious moment, suspended at the top of what was once the largest, deepest gold mine in North America, everything went quiet. Somewhere overhead, the frigid South Dakota winds whistled faintly, whipping through the Black Hills on this February day. A reminder of everything, the whole world we were leaving, as we began to drop. And drop. And drop. The cage moved slowly and steadily, covering about five feet a second. We passed openings to shallower floors, dark and dripping with water. Biologists worked a few of these, scraping up bacteria from the muck, studying extremophi­les to consider life-forms that might exist on other planets. An epic mystery, sure, but our destinatio­n was farther below: floor 4850 in the former Homestake Mine in Lead, outside of Deadwood, South Dakota, now the Sanford Undergroun­d Research Facility, or SURF. Here physicists from around the globe are trying to solve a puzzle more fundamenta­l than life itself. Namely, what is the universe mostly made of? Protons, neutrons, electrons—these are familiar to us. Elementary, even. We have also accounted for the weirder, smaller, subatomic stuff: the alpha and beta particles, the quarks, the neutrinos. Still, they don’t add up. Not by a long shot. In order for existence to, well, exist, for galaxies to spin the way they do, for light from distant stars to bend the way it appears to, there must be quite a bit more out there than we’ve seen so far. The Standard Model, which classifies all elementary particles, accounts for just 16 percent of the universe’s mass. That leaves another 84 percent. There are several theories as to what this remainder might be, but it all goes by the same name: dark matter. The actual nature of dark matter is the subject of much debate. It could be one thing, one type of particle, sort of like a proton; or it could be several different things, like an electron and also a quark. Until we’ve found concrete evidence, we won’t really know for sure. The purpose of the elaborate experiment this cage was descending toward is to find that evidence. Here in the deep, tucked away from the humming interferen­ce radiating from everything around us, sits a wildly complex detector—let’s call it a camera trap. It was designed and built to record the presence of the lead contender for dark matter, a physics unicorn called a WIMP, for weakly interactin­g massive particle. The endeavor includes at its heart a five-foot-tall tank filled with about one-quarter of the world’s annual supply of liquid xenon. If a WIMP passes through, there’s a chance it might glance off a xenon nucleus, which would emit a flash of light, a photon. Once the setup comes online—as soon as late 2020—it will run for five years. At that point the team will have either found proof of a particle that could be dark matter, or...not. The project is known as LUX-ZEPLIN, or LZ. LUX stands for Large Undergroun­d Xenon, ZEPLIN for ZonEd Proportion­al scintillat­ion in LIquid Noble gases. It may well be our best shot yet at spotting a WIMP. “This is the most exciting time for physics, because we still have the really big mysteries in front of us,” says Kevin Lesko, a senior physicist at the Lawrence Berkeley National Laboratory, who coordinate­s the LZ project. In early 2020, the detector was in the final throes of assembly. Teams of six to 12 physicists and engineers worked in two shifts every day, from 8 a.m. to midnight, on an experiment that over five years has required experts in fields as diverse as photon detection and computer modeling, and from some 37 institutio­ns across seven countries. “People like to say we know how to explain the universe. And now we’re trying to figure out the big map of the universe,” Lesko says of the massive collective effort. The xenon tank is the crucial tool for filling in that map by determinin­g what most matter might be. In October 2019, it traveled down the shaft via the cage in a highly orchestrat­ed daylong event that left little room for error or jostling. A single slip and crash, years of planning, plus millions of dollars in research and developmen­t, would have gone down the mine shaft.

THE EVIDENCE OF DARK matter is everywhere, even though we’ve yet to glimpse the stuff itself. In 1933, Fritz Zwicky, a Swiss astronomer based at Caltech, noticed that the velocities of galaxy clusters seemed to make no sense: The gravitatio­nal forces of visible matter wouldn’t be enough to keep the groupings from scattering. For these celestial bodies to congregate the way they do, some unseen mass (plus gravity) must be helping pull them together. In the 1970s, astronomer­s Vera Rubin and Kent Ford were studying the spirals of the Andromeda galaxy and found, to everyone’s astonishme­nt, that the stars at the outermost edges moved just as quickly as those at the center, violating Kepler’s third law of


planetary (in this case, galactic) rotation, which holds that the objects revolving around a core should move more slowly as the distance from the middle increases. That they don’t suggests that some farther-away mass influences these bodies. There are other clues out there, like the way light from remote stars bends on its journey to us, and the consistenc­y of the cosmic microwave background, and the elliptical and spiraling shapes of galaxies. All this points to the existence of a great, nonluminou­s, unseen mass. Peering out into space gives us a sense of the effect dark matter has on the form and appearance of our universe, but all that evidence is indirect, a shadow of a shadow. This invisible stuff will remain a mystery until physicists can observe the particle or particles that account for it, which they’ve been trying to do for about 30 years. Some experiment­s attempt to plot a chart that points to dark matter by searching out evidence of its decay through high-flying instrument­s like the Fermi Gamma-ray Space Telescope. They call this approach indirect detection. Other techniques instead try to create dark matter. Since 2012, physicists have been running experiment­s that could do just that—on the Large Hadron Collider particle accelerato­r at CERN, near Geneva, Switzerlan­d. The efforts, collective­ly called ATLAS, smash together protons to mimic the circumstan­ces of the big bang, when everything in our universe formed, including, theoretica­lly, dark matter. By comparing the energy they know went into the accelerato­r with the measuremen­ts of what comes out, the scientists might prove its existence. More often, dark matter sleuths want hard evidence. That is, they want to directly detect it. But again, no one is precisely sure what it is they’re looking for. Aside from the WIMP, there is another potential culprit: a theoretica­l particle called an axion. If they exist, axions would help explain how neutrons, even those with charged quarks kicking around inside them, manage to remain neutral. They’d also be around one trillion times less massive than an electron, meaning trillions would fit in a space the size of a sugar cube. Physicists think the trick to spying them is speeding up their otherwise glacial decay into photons, which are relatively easy to spot. A detector built by a team at the University of Washington wields a huge and incredibly powerful magnet to hasten that pace, while a resonator tuned to the microwave frequency of the possible decay keeps watch. Yet amid the broad field of dark matter makeups that

scientists have floated over the years— including candidates from primordial black holes to MACHOs (massive astrophysi­cal compact halo objects) half the bulk of our sun—WIMPs have remained a primary target. If they’re out there, they would neatly align with another popular notion in theoretica­l physics called supersymme­try, the idea that for every bit of mass we can see there is also a counterpar­t that is not luminous, the yin to its yang. If that idea’s correct, what we’ve added up from everything covered by the Standard Model would be mirrored by the WIMP presence. The universe, unknowable and chaotic as it may seem, tends toward elegant solutions like this one. Or elegant solutions like this one tend to explain the universe. Still, even within the world of WIMPs, questions remain. The particles might exist in a range of masses, from about one proton to 100,000 protons. One experiment, called SuperCDMS, is searching for wee-er WIMPs than the LZ. Based in a nickel mine in Ontario, Canada, it relies on six detectors made of silicon or germanium crystals; if a WIMP hits one and disturbs a crystal’s electrons, the interactio­n will create vibrations, a signal that can be amplified. The rig runs at –450°F to cut out the noise generated by thermal energy. And it also sits deep undergroun­d—6,800 feet—shielded from the radioactiv­ity of day-to-day life, the cosmic buzz coming off everything from stars to the soles of your Chuck Taylors. There’s another xenon-based WIMP detection attempt, an internatio­nal effort located under Italy’s Gran Sasso mountain—and aptly named XENON. The scientists involved announced in June 2020 that their experiment was registerin­g extra background signals, which could wind up proving there are axions. Or it might be neutrinos. Or the result of contaminat­ion. As with much in dark matter, the data can seem to be on the brink of reality-shifting, but turn out to be nothing at all. Lesko, who’s been working on such subterrane­an experiment­s—including the LZ’s smaller predecesso­r, LUX—for the better part of 30 years, explains why these efforts always happen so deep undergroun­d. Imagine, he says, “you’re trying to hear a whisper. You do it in the middle of New York City, you’re not going to hear it, there’s just too much noise. You want to get away from our background­s—the cosmic rays and junk we’re bombarded by would hide the very rare signals we’re looking for.” But here Lesko stops himself: The signal, the particle, “isn’t necessaril­y rare, what’s rare is the interactio­ns with things we can observe.” The observatio­nal challenges beget a borderline obsession with eliminatin­g every iota of potential interferen­ce. That’s why, when Lesko would fly out to Lead (prepandemi­c, of course) to visit the mine-turned-lab for a week every month, a lot of what he and the crews would work on was keeping absolutely everything as exceptiona­lly clean as possible. It’s a difficult task anywhere, but it’s absurdly so way down inside a tunnel carved into the rock.

THE CAGE DOORS OPENED on level 4850, and we all marched out. Crews of scientists and staff piled into electrifie­d carts—mine carts!— to travel a quarter-mile-long, unlit, dirt-floored passageway toward more distant labs. Closer to where the lift had delivered us, I exchanged my boots for a pair of very clean trail runners that never left this space. I wiped down my phone, pen, notebook, and hands and stepped across a sticky floor to remove any dust from the shoes, then down a long hallway that led to the room where the LZ was coming together. Through the doors came a long, high whistle that sounded like a terrible scream. “That’s the liquid nitrogen we’re running through the pipes—it’s loud!” yelled Aaron Manalaysay, a physicist at the Lawrence Berkeley National Laboratory, over the gassy wails. Manalaysay was down here with a crew of graduate students, working over several months to finish assembling all of LZ’s thousands of component parts, which took up nearly all the room. When the screaming died down, we walked through a set of double doors and into the space. I expected first to see the tank at the center of the LZ experiment, huge and gleaming. Instead there were rows of pipes and wires running from sensors to stacks of computers outside the container; a cryogenics panel for cooling the xenon


gas to just below –163°F (the temperatur­e at which it liquefies) and helping to lower background interferen­ce within the tank itself; plastic curtains draped around areas still undergoing assembly; air ducts and lockers and orange cones and caution signs. At the middle of all this sat a 20-foot-tall, curving stainless-steel structure: the first layer of the LZ’s tank. This would be filled with 70,000 gallons of water to further buffer the inner xenon chamber—in a sense, a gigantic thermos.

Peering into a small, heavy, swung-open porthole revealed the inner sanctum, the xenon tank. Why xenon? It’s extremely dense and, as one of the noble gases, it’s inert. Most of the time, it doesn’t react to most things it comes into contact with. It is, in other words, extremely quiet. So reactions within the element tend to stand out, which is exactly what you want when trying to spot a sudden flash that might end up proving the existence of dark matter. Inside this titanium vessel were photon detectors—the “cameras” in the trap: several hundred 3-inch-wide tubes honeycombe­d into two nearly 5-foot-diameter circles at the top and bottom of the huge canister.

We stepped back from the porthole and climbed a ladder to a mezzanine level midway up the outer tank, where Theresa Fruth, a physics research fellow at University College London, was working on the detectors. She was testing how they would function within the rest of the system. The tubes act as capture and amplificat­ion devices, she explained. When a particle, WIMP or otherwise, moves through the tank and hits the nucleus of a xenon atom, the result is energy, in the form of light: a photon, or more likely many. The arrays absorb these and convert them into electrons. Each one represents a data point—X, Y, and Z coordinate­s—that shows where

in the area an interactio­n is coming from.

The vast majority of the events will stem from the decay in the surroundin­g rock walls. “That will happen,” Fruth said. “We don’t care.” Physicists know what those signals look like and can easily ignore them. Besides, one of the benefits of having such a huge amount of xenon, she explained, is that its outer edges—in addition to the tank itself, and the water, and the other tank, and the mile of earth above—act as a buffer. “If we go closer to the center, we get much quieter.” This was the spot where they might find dark matter. Or where “we can reasonably search for a rare interactio­n.”

ARARE INTERACTIO­N, WERE it to happen inside the tank, could blip without anyone even noticing. The final trick, perhaps the trickiest of all, is to make certain that we do spot this flash of activity amid all the others. Once the LZ comes online, it will register approximat­ely a billion interactio­ns per year. This petabyte worth of data is the responsibi­lity of Maria Elena Monzani. She works at the SLAC National Accelerato­r Laboratory at Stanford and manages the software and computing infrastruc­ture of LUX-ZEPLIN.

Because no one has seen a dark matter interactio­n before, it’s important to try to be sure about everything we have actually seen. Monzani coordinate­s the cataloging and modeling of all the “knowns” in order to make it easier for the unknowns to stand out. “We’re going to have a few billion events, and dark matter will be a handful,” Monzani says. “It’s very important we understand what those few billion events are. Once you know that, then you can know, ‘Ah, this is something.’”

Monzani oversees what is, in essence, an inoculatio­n against the mind’s urge to see things (patterns, particles) that aren’t really there. She’s got several platoons’ worth of physicists spread around the globe, working on two data centers running full simulation­s of the LZ. They’re calibratin­g the machine, the algorithms, and, yes, the humans. To calibrate a person, Monzani and her team churn out datasets from a simulation of the LZ tank, then, diabolical­ly, add extra data that looks just like the real thing—a method called salting.

Monzani’s crew drops in data that, say, looks like the energy a WIMP would leave in its wake. They know these markers are fake, but their analysts don’t, thus creating a blind test to reduce the bias that may come about from physicists’ very real desire to find an exciting interactio­n. When the trial run is done, Monzani’s team reveals which of the signals were placebos. What’s left is, in this case, the “real” ones created by the LZ simulation (they’ll repeat the process when the experiment turns on and live data starts coming in). Everyone wants to find dark matter. Salting trains them to be honest.

Running simulation after simulation of the LZ systems became the bulk of the

effort in midspring. In March 2020, the COVID-19 pandemic forced the facility to shut down on-site work aside from critical maintenanc­e. Some of the scientists stayed in town, since travel— particular­ly internatio­nally—seemed dicey, and Lead (population 3,021) was a pleasant enough place to be stranded for however long they would hang in this virus-induced limbo.

There’s still plenty to do abovegroun­d, plenty of calibratio­ns to perfect. No matter when they start, it’ll take five years of WIMP sniffing to gather enough data to know if the particle is in the LZ’s detection range. And besides, as project coordinato­r Lesko points out, all those months of double shifts had paid off: They’d nearly completed assembly down on 4850, and the project was in a stable and safe spot. Few places are more secure during a pandemic than one close to a mile undergroun­d.

Still, like the rest of us, they wonder when this all might be over: when they can get fully back to the experiment, and if, once they do—with the LZ tank sealed and detector arrays watchfully waiting—they’ll find anything at all. None of the nearly one dozen prior attempts to nab a glimmer of a WIMP over the past three decades have worked. Yet team members like Fruth, the photon detector specialist, are sanguine about the possibilit­y of their life’s work netting nothing. “Knowing that it’s not something is still worth something,” she says. When you aren’t sure exactly what a WIMP is, there’s value in finding out what it isn’t.

Living with uncertaint­y and pondering the unknown is a comfortabl­e space for them to be in, because that’s what scientists do—especially physicists on this particular ongoing hunt. Fruth likens dark matter to the unfilled portion of a map, the “here be monsters” bits. “We draw this line,” she says, “and we say, ‘Look, we don’t know anything beyond this line.’ And then we push a little farther, and know a little more. And the line moves, and we move with it.”

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 ??  ?? ABOVE: A tank filled with xenon awaits transport to its home in a former gold mine below Lead, South Dakota. LEFT: Mine carts transport new kinds of workers and equipment.
ABOVE: A tank filled with xenon awaits transport to its home in a former gold mine below Lead, South Dakota. LEFT: Mine carts transport new kinds of workers and equipment.
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 ??  ?? ABOVE: The sensors of a single array; two of them peer into the vessel from above and below.
ABOVE: The sensors of a single array; two of them peer into the vessel from above and below.
 ??  ?? ABOVE: The LZ’s xenon tank slips into its housing. LEFT: The Black Hills flank Lead, South Dakota. BELOW: The crew watches as the tank lowers into the spot where it might finally nab dark matter.
ABOVE: The LZ’s xenon tank slips into its housing. LEFT: The Black Hills flank Lead, South Dakota. BELOW: The crew watches as the tank lowers into the spot where it might finally nab dark matter.
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