Glimpse the gold mine the place scientists are looking for darkish matter

For many years, physicists have been attempting to spy darkish matter. An elaborate digital camera entice would possibly quickly glimpse it for the primary time. (Matthew Kapust/Sanford Underground Analysis Facility/)

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There was a pause, simply earlier than the cage doorways got here rattling closed and we started our 15-minute descent 4,850 toes into the earth. We have been packed in tight: a crew of some 30 physicists, engineers, biologists, and—principally—miners. Ex-miners, really. This hadn’t been an energetic mine for 18 years. The man working the elevate let the winch operator above us know that the cab was full, that we have been a go. For a short, delirious second, suspended on the high of what was as soon as the biggest, deepest gold mine in North America, all the pieces went quiet. Someplace overhead, the frigid South Dakota winds whistled faintly, whipping by way of the Black Hills on this February day. A reminder of all the pieces, the entire world we have been leaving, as we started to drop.

And drop.

And drop.

The cage moved slowly and steadily, masking about 5 toes a second. We handed openings to shallower flooring, darkish and dripping with water. Biologists labored just a few of those, scraping up micro organism from the muck, finding out extremophiles to think about life-forms which may exist on different planets. An epic thriller, positive, however our vacation spot was farther under: ground 4850 within the former Homestake Mine in Lead, exterior of Deadwood, South Dakota, now the Sanford Underground Analysis Facility, or SURF. Right here physicists from across the globe try to resolve a puzzle extra basic than life itself. Specifically, what’s the universe principally made from?

A tank that will be filled with xenon awaits transport to its home in a former gold mine below Lead, South Dakota.

A tank that will likely be crammed with xenon awaits transport to its house in a former gold mine under Lead, South Dakota. (Nick Hubbard/Sanford Underground Analysis Facility/)

Protons, neutrons, electrons—these are acquainted to us. Elementary, even. Now we have additionally accounted for the weirder, smaller, subatomic stuff: the alpha and beta particles, the quarks, the neutrinos. Nonetheless, they don’t add up. Not by a protracted shot. To ensure that existence to, properly, exist, for galaxies to spin the best way they do, for mild from distant stars to bend the best way it seems to, there have to be fairly a bit extra on the market than we’ve seen to date. The Normal Mannequin, which classifies all elementary particles, accounts for simply 16 % of the universe’s mass. That leaves one other 84 %. There are a number of theories as to what this the rest could be, nevertheless it all goes by the identical title: darkish matter.

The precise nature of darkish matter is the topic of a lot debate. It might be one factor, one kind of particle, form of like a proton; or it might be a number of various things, like an electron and likewise a quark. Till we’ve discovered concrete proof, we received’t actually know for positive. The aim of the flowery experiment this cage was descending towards is to seek out that proof.

Right here within the deep, tucked away from the buzzing interference radiating from all the pieces round us, sits a wildly advanced detector—let’s name it a digital camera entice. It was designed and constructed to report the presence of the lead contender for darkish matter, a physics unicorn referred to as a WIMP, for weakly interacting large particle. The endeavor contains at its coronary heart a five-foot-tall tank crammed with about one-quarter of the world’s annual Source of liquid xenon. If a WIMP passes by way of, there’s an opportunity it’d look off a xenon nucleus, which might emit a flash of sunshine, a photon. As soon as the setup comes on-line—as quickly as late 2020—it should run for 5 years. At that time the group may have both discovered proof of a particle that might be darkish matter, or…not. The venture is called LUX-ZEPLIN, or LZ. LUX stands for Giant Underground Xenon, ZEPLIN for ZonEd Proportional scintillation in LIquid Noble gases. It could be our greatest shot but at recognizing a WIMP.

“That is essentially the most thrilling time for physics, as a result of we nonetheless have the actually large mysteries in entrance of us,” says Kevin Lesko, a senior physicist on the Lawrence Berkeley Nationwide Laboratory, who coordinates the LZ venture. In early 2020, the detector was within the last throes of meeting. Groups of six to 12 physicists and engineers labored in two shifts every single day, from eight a.m. to midnight, on an experiment that over 5 years has required specialists in fields as numerous as photon detection and laptop modeling, and from some 37 establishments throughout seven international locations. “Folks prefer to say we all know clarify the universe. And now we’re attempting to determine the large map of the universe,” Lesko says of the large collective effort.

The xenon tank is the essential software for filling in that map by figuring out what most matter could be. In October 2019, it traveled down the shaft by way of the cage in a extremely orchestrated daylong occasion that left little room for error or jostling. A single slip and crash, years of planning, plus tens of millions of {dollars} in analysis and improvement, would have gone down the mine shaft.

Mine carts transport new kinds of workers and equipment.

Mine carts transport new sorts of employees and tools. (Ryan Bradley/)

The proof of darkish matter is in every single place, despite the fact that we’ve but to glimpse the stuff itself. In 1933, Fritz Zwicky, a Swiss astronomer primarily based at Caltech, observed that the velocities of galaxy clusters appeared to make no sense: The gravitational forces of seen matter wouldn’t be sufficient to maintain the groupings from scattering. For these celestial our bodies to congregate the best way they do, some unseen mass (plus gravity) have to be serving to pull them collectively. Within the 1970s, astronomers Vera Rubin and Kent Ford have been finding out the spirals of the Andromeda galaxy and located, to everybody’s astonishment, that the celebs on the outermost edges moved simply as rapidly as these on the heart, violating Kepler’s third legislation of planetary (on this case, galactic) rotation, which holds that the objects revolving round a core ought to transfer extra slowly as the space from the center will increase. That they don’t means that some farther-away mass influences these our bodies. There are different clues on the market, like the best way mild from distant 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 factors to the existence of a fantastic, nonluminous, unseen mass.

Peering out into area offers us a way of the impact darkish matter has on the shape and look of our universe, however all that proof is oblique, a shadow of a shadow. This invisible stuff will stay a thriller till physicists can observe the particle or particles that account for it, which they’ve been attempting to do for about 30 years. Some experiments try to plot a chart that factors to darkish matter by looking for proof of its decay by way of high-flying devices just like the Fermi Gamma-ray House Telescope. They name this strategy oblique detection.

Different strategies as a substitute attempt to create darkish matter. Since 2012, physicists have been operating experiments that might just do that—on the Giant Hadron Collider particle accelerator at CERN, close to Geneva, Switzerland. The efforts, collectively referred to as ATLAS, smash collectively protons to imitate the circumstances of the large bang, when all the pieces in our universe shaped, together with, theoretically, darkish matter. By evaluating the power they know went into the accelerator with the measurements of what comes out, the scientists would possibly show its existence.

Physicists wire up a ­photon-​­detector array to transmit potential WIMP signals out of the xenon tank.

Physicists wire up a ­photon-​­detector array to transmit potential WIMP indicators out of the xenon tank. (Matthew Kapust/Sanford Underground Analysis Facility/)

Extra typically, darkish matter sleuths need laborious proof. That’s, they wish to immediately detect it. However once more, nobody is exactly positive what it’s they’re on the lookout for. Except for the WIMP, there may be one other potential wrongdoer: a theoretical particle referred to as an axion. In the event that they exist, axions would assist clarify how neutrons, even these with charged quarks kicking round inside them, handle to stay impartial. They’d even be round one trillion occasions much less large than an electron, which means trillions would slot in an area the scale of a sugar dice. Physicists suppose the trick to spying them is rushing up their in any other case glacial decay into photons, that are comparatively straightforward to identify. A detector constructed by a group on the College of Washington wields an enormous and extremely highly effective magnet to hasten that tempo, whereas a resonator tuned to the microwave frequency of the potential decay retains watch.

But amid the broad discipline of darkish matter makeups that scientists have floated through the years—together with candidates from primordial black holes to MACHOs (large astrophysical compact halo objects) half the majority of our solar—WIMPs have remained a main goal. In the event that they’re on the market, they might neatly align with one other fashionable notion in theoretical physics referred to as supersymmetry, the concept for each little bit of mass we will see there may be additionally a counterpart that isn’t luminous, the yin to its yang. If that concept’s appropriate, what we’ve added up from all the pieces lined by the Normal Mannequin could be mirrored by the WIMP presence. The universe, unknowable and chaotic as it might appear, tends towards elegant options like this one. Or elegant options like this one have a tendency to elucidate the universe.

Nonetheless, even throughout the world of WIMPs, questions stay. The particles would possibly exist in a variety of lots, from about one proton to 100,000 protons. One experiment, referred to as SuperCDMS, is looking for wee-er WIMPs than the LZ. Primarily based in a nickel mine in Ontario, Canada, it depends on six detectors made from silicon or germanium crystals; if a WIMP hits one and disturbs a crystal’s electrons, the interplay will create vibrations, a sign that may be amplified. The rig runs at –450°F to chop out the noise generated by thermal power. And it additionally sits deep underground—6,800 toes—shielded from the radioactivity of day-to-day life, the cosmic buzz coming off all the pieces from stars to the soles of your Chuck Taylors.

The sensors of a single array; two of them peer into the vessel from above and below.

The sensors of a single array; two of them peer into the vessel from above and under. (Matthew Kapust/Sanford Underground Analysis Facility/)

There’s one other xenon-based WIMP detection try, a global effort situated beneath Italy’s Gran Sasso mountain—and aptly named XENON. The scientists concerned introduced in June 2020 that their experiment was registering further background indicators, which may wind up proving there are axions. Or it could be neutrinos. Or the results of contamination. As with a lot in darkish matter, the info can appear to be on the point of reality-shifting, however grow to be nothing in any respect.

Lesko, who’s been engaged on such subterranean experiments—together with the LZ’s smaller predecessor, LUX—for the higher a part of 30 years, explains why these efforts all the time occur so deep underground. Think about, he says, “you’re attempting to listen to a whisper. You do it in the course of New York Metropolis, you’re not going to listen to it, there’s simply an excessive amount of noise. You wish to get away from our backgrounds—the cosmic rays and junk we’re bombarded by would conceal the very uncommon indicators we’re on the lookout for.” However right here Lesko stops himself: The sign, the particle, “isn’t essentially uncommon, what’s uncommon is the interactions with issues we will observe.”

The observational challenges beget a borderline obsession with eliminating each iota of potential interference. That’s why, when Lesko would fly out to Lead (pre-pandemic, in fact) to go to the mine-turned-lab for every week each month, numerous what he and the crews would work on was protecting completely all the pieces as exceptionally clear as potential. It’s a tough activity anyplace, nevertheless it’s absurdly so means down inside a tunnel carved into the rock.

The LZ’s xenon tank slips into its housing.

The LZ’s xenon tank slips into its housing. (Nick Hubbard/Sanford Underground Analysis Facility/)

The cage doorways opened on stage 4850, and all of us marched out. Crews of scientists and employees piled into electrified carts—mine carts!—to journey a quarter-mile-long, unlit, dirt-floored passageway towards extra distant labs. Nearer to the place the elevate had delivered us, I exchanged my boots for a pair of very clear path runners that by no means left this area. I wiped down my cellphone, pen, pocket book, and fingers and stepped throughout a sticky ground to take away any mud from the sneakers, then down a protracted hallway that led to the room the place the LZ was coming collectively. By means of the doorways got here a protracted, excessive whistle that gave the impression of a horrible scream.

“That’s the liquid nitrogen we’re operating by way of the pipes—it’s loud!” yelled Aaron Manalaysay, a physicist on the Lawrence Berkeley Nationwide Laboratory, over the gassy wails. Manalaysay was down right here with a crew of graduate college students, working over a number of months to complete assembling all of LZ’s 1000’s of element elements, which took up almost all of the room.

When the screaming died down, we walked by way of a set of double doorways and into the area. I anticipated first to see the tank on the heart of the LZ experiment, enormous and gleaming. As a substitute there have been rows of pipes and wires operating from sensors to stacks of computer systems exterior the container; a cryogenics panel for cooling the xenon fuel to simply under –163°F (the temperature at which it liquefies) and serving to to decrease background interference throughout the tank itself; plastic curtains draped round areas nonetheless present process meeting; air ducts and lockers and orange cones and warning indicators. On the center of all this sat a 20-foot-tall, curving stainless-steel construction: the primary layer of the LZ’s tank. This may be crammed with 70,000 gallons of water to additional buffer the inside xenon chamber—in a way, a huge thermos.

<b>How to spot dark matter.</b> The quest to capture proof of WIMPs, a lead candidate for the makeup of dark matter, sends the physicists behind the LUX-ZEPLIN experiment below ground. Nearly a mile down, background noise is minimal, but spying the particles still requires a rig looking for a very specific interaction.
<br><I>A. Set a target</i>: Dark matter particles, possibly WIMPS, surround and move through everything, the LZ included, even if we don’t notice them.
<br><I>B. Eliminate noise</i>: To up their chances of spotting one, physicists buffer the WIMP trap with many layers, the innermost of which is a titanium tank.
<br><I>C. Wait quietly</i>: Nonreactive, some 11 tons of liquid xenon housed within the tank creates a placid space in which to watch for dark matter activity.
<br><I>D. Interact</i>: Should a WIMP glance off the nucleus of a ­xenon atom, the collision will elicit sparks of energy: a burst of photons.
<br><I>E. Grab the flash</i>: Hundreds of 3-inch-wide photon detectors nestled into circles at the top and bottom of the structure amplify any activity.
<br><I>F. Record the signal</i>: The array ­converts the burst into ­electrons—data points that indicate the spot within the tank where the ­interaction occurred.

<b>How you can spot darkish matter.</b> The hunt to seize proof of WIMPs, a lead candidate for the make-up of darkish matter, sends the physicists behind the LUX-ZEPLIN experiment under floor. Practically a mile down, background noise is minimal, however spying the particles nonetheless requires a rig on the lookout for a really particular interplay.
<br><I>A. Set a goal</i>: Darkish matter particles, presumably WIMPS, encompass and transfer by way of all the pieces, the LZ included, even when we don’t discover them.
<br><I>B. Remove noise</i>: To up their possibilities of recognizing one, physicists buffer the WIMP entice with many layers, the innermost of which is a titanium tank.
<br><I>C. Wait quietly</i>: Nonreactive, some 11 tons of liquid xenon housed throughout the tank creates a placid area through which to look at for darkish matter exercise.
<br><I>D. Work together</i>: Ought to a WIMP look off the nucleus of a ­xenon atom, the collision will elicit sparks of power: a burst of photons.
<br><I>E. Seize the flash</i>: Tons of of 3-inch-wide photon detectors nestled into circles on the high and backside of the construction amplify any exercise.
<br><I>F. Document the sign</i>: The array ­converts the burst into ­electrons—information factors that point out the spot throughout the tank the place the ­interplay occurred. (Maxwell Erwin/)

Peering right into a small, heavy, swung-open porthole revealed the inside sanctum, the xenon tank. Why xenon? It’s extraordinarily dense and, as one of many noble gases, it’s inert. More often than not, it doesn’t react to most issues it comes into contact with. It’s, in different phrases, extraordinarily quiet. So reactions throughout the ingredient have a tendency to face out, which is strictly what you need when attempting to identify a sudden flash which may find yourself proving the existence of darkish matter. Inside this titanium vessel have been photon detectors—the “cameras” within the entice: a number of hundred 3-inch-wide tubes honeycombed into two almost 5-foot-diameter circles on the high and backside of the massive canister.

We stepped again from the porthole and climbed a ladder to a mezzanine stage halfway up the outer tank, the place Theresa Fruth, a physics analysis fellow at College Faculty London, was engaged on the detectors. She was testing how they might operate inside the remainder of the system. The tubes act as seize and amplification gadgets, she defined. When a particle, WIMP or in any other case, strikes by way of the tank and hits the nucleus of a xenon atom, the result’s power, within the type of mild: a photon, or extra seemingly many. The arrays soak up these and convert them into electrons. Each represents a knowledge level—X, Y, and Z coordinates—that exhibits the place within the space an interplay is coming from.

The overwhelming majority of the occasions will stem from the decay within the surrounding rock partitions. “That may occur,” Fruth stated. “We don’t care.” Physicists know what these indicators seem like and might simply ignore them. In addition to, one of many advantages of getting such an enormous quantity of xenon, she defined, is that its outer edges—along with the tank itself, and the water, and the opposite tank, and the mile of earth above—act as a buffer. “If we go nearer to the middle, we get a lot quieter.” This was the spot the place they may discover darkish matter. Or the place “we will moderately seek for a uncommon interplay.”

The Black Hills flank Lead, South Dakota.

The Black Hills flank Lead, South Dakota. (Nick Hubbard/Sanford Underground Analysis Facility/)

A uncommon interplay, have been it to occur contained in the tank, may blip with out anybody even noticing. The ultimate trick, maybe the trickiest of all, is to make sure that we do spot this flash of exercise amid all of the others. As soon as the LZ comes on-line, it should register roughly a billion interactions per 12 months. This petabyte price of knowledge is the duty of Maria Elena Monzani. She works on the SLAC Nationwide Accelerator Laboratory at Stanford and manages the software program and computing infrastructure of LUX-ZEPLIN.

As a result of nobody has seen a darkish matter interplay earlier than, it’s essential to attempt to make certain about all the pieces now we have really seen. Monzani coordinates the cataloging and modeling of all of the “knowns” with the intention to make it simpler for the unknowns to face out. “We’re going to have just a few billion occasions, and darkish matter will likely be a handful,” Monzani says. “It’s crucial we perceive what these few billion occasions are. As soon as you realize that, then you may know, ‘Ah, that is one thing.’”

Monzani oversees what’s, in essence, an inoculation towards the thoughts’s urge to see issues (patterns, particles) that aren’t actually there. She’s acquired a number of platoons’ price of physicists unfold across the globe, engaged on two information facilities operating full simulations of the LZ. They’re calibrating the machine, the algorithms, and, sure, the people. To calibrate an individual, Monzani and her group churn out datasets from a simulation of the LZ tank, then, diabolically, add further information that appears similar to the actual factor—a way referred to as salting.

Monzani’s crew drops in information that, say, appears to be like just like the power a WIMP would go away in its wake. They know these markers are pretend, however their analysts don’t, thus making a blind take a look at to scale back the bias which will come about from physicists’ very actual need to seek out an thrilling interplay. When the trial run is finished, Monzani’s group reveals which of the indicators have been placebos. What’s left is, on this case, the “actual” ones created by the LZ simulation (they’ll repeat the method when the experiment activates and reside information begins coming in). Everybody desires to seek out darkish matter. Salting trains them to be sincere.

The crew watches as the tank lowers into the spot where it might finally nab dark matter.

The crew watches because the tank lowers into the spot the place it’d lastly nab darkish matter. (Nick Hubbard/Sanford Underground Analysis Facility/)

Working simulation after simulation of the LZ methods grew to become the majority of the hassle in midspring. In March 2020, the COVID-19 pandemic compelled the ability to close down on-site work other than crucial upkeep. Among the scientists stayed on the town, since journey—significantly internationally—appeared dicey, and Lead (inhabitants 3,021) was a pleasing sufficient place to be stranded for nonetheless lengthy they might grasp on this virus-induced limbo.

There’s nonetheless loads to do aboveground, loads of calibrations to excellent. Irrespective of once they begin, it’ll take 5 years of WIMP sniffing to assemble sufficient information to know if the particle is within the LZ’s detection vary. And in addition to, as venture coordinator Lesko factors out, all these months of double shifts had paid off: They’d almost accomplished meeting down on 4850, and the venture was in a secure and protected spot. Few locations are safer throughout a pandemic than one near a mile underground.

Nonetheless, like the remainder of us, they surprise when this all could be over: once they can get totally again to the experiment, and if, as soon as they do—with the LZ tank sealed and detector arrays watchfully ready—they’ll discover something in any respect. Not one of the almost one dozen prior makes an attempt to nab a glimmer of a WIMP over the previous three a long time have labored. But group members like Fruth, the photon detector specialist, are sanguine about the potential of their life’s work netting nothing. “Understanding that it’s not one thing continues to be price one thing,” she says. While you aren’t positive precisely what a WIMP is, there’s worth find out what it isn’t.

Dwelling with uncertainty and pondering the unknown is a cushty area for them to be in, as a result of that’s what scientists do—particularly physicists on this explicit ongoing hunt. Fruth likens darkish matter to the unfilled portion of a map, the “right here be monsters” bits. “We draw this line,” she says, “and we are saying, ‘Look, we don’t know something past this line.’ After which we push a bit farther, and know a bit extra. And the road strikes, and we transfer with it.”

This story seems within the Fall 2020, Mysteries challenge of Well-liked Science.

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