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In a Sudbury Mine, Physicists Probe Secrets of Dark Matter

Dec 8, 2015

SUDBURY, ONT.—In a hot, dark cavern buried two kilometres below the earth’s surface, a pallet of No Name dog food lies covered in dust.
These subterranean passageways have certainly seen stranger sights than bulk dog food. There was the one-of-a-kind sanding robot, for starters. There was the giant acrylic orb, split in two pieces to fit down the mine’s narrow elevator shaft. Over the next four weeks, there will be 3.6 tonnes of liquid argon.
Every day, a parade of physicists in coveralls and head lamps rattles down the elevator and tramps through these passages — plus engineers, welders, machinists, grad students, the occasional journalist. Stephen Hawking was here.
But to grasp the scale and ambition of what’s happening at SNOLAB, it helps to think about that pallet of dog food.
The scientists down here are building a massive experiment, DEAP-3600, designed to capture faint signals from dark matter, one of the greatest unresolved mysteries in physics. Whatever dark matter is, it accounts for the vast majority of the matter in the universe. Physicists have described the ordinary, visible matter we know — galaxies, comets, planets, us — as the froth on top of a deep, dark ocean. But we don’t know what that ocean is made of. Dark matter is invisible: its existence is inferred, never seen.
At SNOLAB, scientists want to change that. They are building the world’s most sensitive dark matter detector of its kind, going to painstaking lengths — burying the lab in an ore mine in Sudbury, for instance — to avoid anything that might mask a signal.
An experiment of this scale is a scientific feat involving 65 researchers at 10 institutions in three countries. It is also a logistical nightmare.
“We’re pushing right at the edge of technical capabilities of different scientific techniques,” says Mark Boulay, an experimental particle astrophysicist at Carleton and Queen’s universities and project director for DEAP. “But we’re also building a large construction project.”
On top of the behaviour of subatomic particles, Boulay and his DEAP collaborators must contend with Ministry of Labour approvals, missing wrenches, and budgets, budgets, budgets. Someone at SNOLAB must maintain that large supply of dog food. The lab hosts dozens of workers daily, but usually not enough to satisfy the microbes that keep the sewer treatment plant functional. Dog food supplements the microbes’ diet.
These prosaic demands can seem jarring in contrast to the lab’s and the experiment’s ambitions. On Thursday, Queen’s University physicist Arthur McDonald will accept a Nobel Prize for his work on the Sudbury Neutrino Observatory (SNO). SNO was the precursor to the expanded SNOLAB, where 10 experiments are now underway in addition to DEAP. Boulay was part of the SNO team; DEAP is the inheritance of the expertise accumulated as a direct result of its success.
“Certainly with the facility we have at SNOLAB, and all the expertise we have built up in Canada in particle astrophysics, we are at the leading edge of the field. What we are doing is of that calibre,” says Boulay. “We have excellent potential for discovery and for scientific impact, and we are right around the corner from turning on.”
But experimental particle physics is big, high-stakes science. Other ambitious dark matter detectors have found nothing, which is helpful for defining where to look next, but not the result researchers dream about. If theorists’ current best guess for what dark matter is made of is wrong, DEAP won’t find anything either.
Then again, if the theorists are right, the world’s best shot at discovering dark matter may be sitting in an ore mine in Sudbury.

WIMPs that go bump in the night
To get to work every day, SNOLAB scientists and staff perform what is surely one of the world’s strangest commutes.
Usually before dawn, they arrive at Creighton Mine, a half-hour drive west of downtown Sudbury. Creighton is an active ore mine owned by Vale (formerly Inco). Vale allows the scientists to piggyback on its existing infrastructure, a critical resource: without it, operating the lab would cost millions more. At Creighton, the scientists and staff suit up in mining gear: coveralls, head lamps, safety belts.
They cram shoulder to shoulder with miners in “the cage,” an open-sided elevator. After a rat-a-tat Morse code-like message to the operator below, the cage starts to plummet down the mine shaft. It will descend two kilometres — almost four times the length of the CN tower — so quickly that newbies are advised to chew gum.
At the second-deepest stop, the SNOLAB scientists are released into “the drift,” a dark, dust-flecked tunnel. The ambient temperature in the drift is 42 C — ventilation lowers it — and the air pressure is 20 per cent higher than at surface, a combination of effects that can leave a first-timer feeling slightly strange. The ground is muddy and criss-crossed by railcar tracks.
After trudging 1.4 kilometres through the drift, the crew arrives at a door and a wall of hoses hammered into the rock: the boot wash station. “Welcome to SNOLAB,” a banner declares. “Your cleanliness journey begins here!” The banner marks a transition in this commute: the switch between the dirty first half and the even more convoluted, clean second half.
Why bury a physics lab in an operational ore mine? Because every minute on the Earth’s surface, thousands of super-high-energy particles from outer space bombard your body. This is not a tinfoil-hat conspiracy. It is a basic fact of physics.
These “cosmic rays” were great for mid-century scientists, who measured them to discover subatomic particles. They are harmless for the rest of us, part of the background radiation we absorb daily. They are ruinous for a dark matter detector.
DEAP relies on picking up incredibly faint interactions — if they are happening at all — between dark matter and the 3.6 tonnes of liquid argon trapped inside an acrylic vessel at its core. Again, physicists have no idea what dark matter is made of. But the most popular candidate is a hypothetical particle known as a WIMP, for weakly interacting massive particle. The SNOLAB scientists are hoping to see a WIMP bump into the nucleus of an argon atom, emitting a pulse of light that the detector can capture.
Above ground, cosmic rays would ping the argon incessantly, overwhelming the dark matter interactions scientists are looking for. Burying the lab in a mine underneath 2,070 metres of norite rock substantially reduces this problem: a dozen or fewer particles will make it through the rock every month. But cosmic rays are not the only type of radiation that keeps Boulay up at night, not even close.
The potassium in human sweat is slightly radioactive; half a dozen fingerprints would jeopardize the experiment. But the “worst enemy” of detectors is radon, a radioactive gas that is the decay product of uranium and thorium. Radon is found naturally in all kinds of environments, including soil, rock and air. It can reach levels dangerous to human health if it becomes trapped in an enclosed space, like a well-sealed basement. Radon is found in particularly high dosages in mines.
You can probably anticipate the irony here. A crucial part of what will make DEAP the most sensitive dark matter detector of its kind is its ultra-clean environment: the scientists’ ability to mute background noise, or unwanted interactions. Burying the lab in an ore mine accomplishes that in part. But burying the lab in an ore mine also makes the risk of exposure to other types of radiation substantially worse.
“You can think of the mine down here as sort of the deepest, darkest, most well-sealed basement — the worst place ever for radon,” says Boulay.
To reduce contamination from the mine, everything that enters SNOLAB — including the people — follows a strict routine belied by the cheery tone of the boot wash station banner.

No detail is ‘trivial’
In matching blue onesies and white helmets, the staff of SNOLAB sometimes resembles a diligent Smurf colony.
The outfits are part of a stringent cleanliness protocol that begins after entering SNOLAB from the drift, including showering, changing into a laundered set of clothing that never leaves the facility, and donning hairnets and a clean helmet.
Everything else that enters SNOLAB is run through a room called the “car wash,” which is exactly what it sounds like. Inside SNOLAB, the walls are covered in four inches of shotcrete and painted with a glossy, easily washable material; sticky, dust-trapping mats lie underfoot.
“Every single surface has been cleaned by hand,” says Nigel Smith, SNOLAB’s director. “Every nut and bolt and piece of steel or bracket gets washed and cleaned before it comes into the lab.”
If this sounds exacting, it’s nothing compared to the rigour with which the scientists select materials that make up DEAP.
“People in our field do some really mad stuff, generally, to find low-background materials,” says Smith. “There’s no point coming all the way down here and shielding your detector and then putting a radioactive component into (it).”
The plumbing system that will draw the argon into the acrylic vessel at the detector’s core is made of electro-polished stainless steel, a process that involves submerging the steel in a vat of acid and running an electrical current through it. Electro-polishing removes a thin layer of surface material, making the steel incredibly smooth and easy to clean.
The argon itself will be purged of radon through a custom-built, low-radioactivity charcoal filter. Arthur McDonald is leading DEAP’s search for purer forms of argon, and is collaborating with a U.S. group that — for the benefit of obsessive experimental physicists — is hunting for argon from deep underground sources, which contain less of a troublesome isotope produced via interactions with cosmic rays. This ultra-pure argon will be used in later runs of the experiment, boosting the detector’s sensitivity.
From experience with the Sudbury Neutrino Observatory, the team already knew that acrylic is an exceptionally clean material. It is usually used in environments where it needs to be visibly clear: the primary business of one company SNOLAB works with is fabricating massive tanks for aquariums and zoos. The vessel fabricated for DEAP, the team claims, is made of the cleanest acrylic ever manufactured.
DEAP collaborators travelled to the facility in Thailand where the acrylic panels were cast to scrutinize the process: mixing a monomer slurry, pouring it into moulds, and letting it cure. Special air filters were installed in the factory, and the transport trucks followed a strict protocol. Afterward, the 11-centimetre-thick panels were shipped to Colorado, where they were heated and bent into five orange-slice-shaped sections and bonded together. The vessel was machined by DEAP collaborators at the University of Alberta and then transported to Creighton, where it was slung below the cage — it was too big to fit inside — and carefully lowered down.
That wasn’t enough trouble for the team: Queen’s engineers and scientists spent five years designing and building a resurfacing robot to shave approximately a millimetre of acrylic from the inner surface of the vessel, which may have been contaminated with radon simply from being exposed to air. The robot used diamond sanding pads that were chosen like many other detector materials: by testing a dozen choices in a radon assay system and selecting the one with the lowest levels. After sanding, the interior was flushed with tonne after tonne of ultra-pure water (kind of like tooth polishing, as a DEAP team member suggested).
“We are trying to build some of the lowest-radiation environments in the universe,” Smith says. This is what makes DEAP 20 times more sensitive than the next best dark matter detector.
These science concerns are always compounded by logistical ones. Boulay’s most frequently used expression is “non-trivial,” and he applies it to many things. Removing the sanding robot from the interior of the acrylic vessel? Non-trivial. It involved installing an extraction canister, which involved operating a lifting device, which involved waiting for an approval, one of the many delays DEAP has experienced (though it is not as far behind schedule as SNO was).
“Doing things that haven’t been done before is not trivial,” Boulay says. “We’re doing them at a very large scale, and we’re doing them underground, which complicates things enormously.”
“It’s critical because if we drop it, we’re screwed,” Smith said three months earlier, explaining why the entire DEAP team was in meetings on the surface. (He quickly clarified that “critical” technically means lifting something close to the maximum capacity of the hoist.)
By the time of the lift, the vessel and its frame weighed 30,000 pounds. It was covered in hardware, including 255 photomultipliers, which collect the light generated by a dark matter event. A team member who laid his hands on it to check a load sensor looked as though he was trying to perform a religious miracle. But after months of meetings and two four-inch-thick binders of plans, the critical lift was a success. The vessel now hangs in an eight-metre-wide tank of ultra-pure water, its final protective shield.
The team will spend the next several weeks running calibrations that, when the detector begins to collect data early next year, will help them differentiate between false events and real dark matter detections. Despite the DEAP team’s incredible diligence, the detector will still be drowned in a cacophony of noise: in a single year, it might register a dozen dark matter detections compared to a billion background events.
But if Boulay and his collaborators see what they are looking for, it will be the resolution of a tantalizing cosmic mystery.

Deep dark secrets
Dark matter is just the latest insult to the notion that humans and our tiny blue planet are central to the universe, as Ken Freeman and Geoff McNamara write in their book In Search of Dark Matter.
Copernicus showed that the sun, not the Earth, is the centre of our solar system. Galileo discovered that our sun is just one among many in the Milky Way. Edwin Hubble saw that the Milky Way was not the entirety of the universe but one galaxy among many.
Now, we know that everything we are made of and everything we can see — visible matter; matter that interacts with the electromagnetic force and therefore reflects, absorbs and emits light — is an insignificant fraction of the mass in the universe, less than 5 per cent.
Dark matter accounts for 26 per cent of everything in the universe. The rest is dark energy, an even more mysterious phenomenon.
Astronomers first began stumbling against dark matter in the 1930s. Fritz Zwicky, a Swiss-born astronomer working in California, is known today as both an underappreciated genius and an irascible oddball (he reportedly referred to his many academic enemies as “spherical bastards,” because they were bastards viewed from any angle). Zwicky hypothesized the existence of dark matter when he noticed that galaxies in the distant Coma Cluster were spinning far too quickly considering how much they weighed. His unpopularity may have been part of the reason his ideas didn’t gain widespread acceptance before his death in 1974.
But Rubin and Ford didn’t observe that. The outer stars were spinning as quickly as the interior stars, and sometimes faster. They concluded that the galaxies must be surrounded by a halo of matter they could not see.
Theorists have postulated many candidates for what dark matter might be. But the most widely accepted hypothesis is the WIMP, a particle that interacts with gravity but not light, hence its invisibility to us.
Neither WIMPs nor any other particle that could successfully explain dark matter exist in the standard model of particle physics, the theoretical framework that has successfully predicted nearly all the phenomena in the universe.
Directly observing a WIMP interaction would not be the first advance in physics “beyond the standard model”: the discovery that neutrinos oscillate, for which Arthur McDonald co-won the Nobel Prize, showed that the standard model cannot be complete.
But observing dark matter would open up a new chapter in physics. It would almost certainly earn another Nobel Prize for SNOLAB — or whoever finds it first. Other detectors are running or underway, including one inside a mountain in Italy that has similar sensitivities to DEAP but is scheduled to turn on later and uses xenon. Experiments are ongoing at the Large Hadron Collider and aboard the International Space Station.
Asked whether he would be disappointed if DEAP did not detect dark matter, project director Mark Boulay hesitates, then sighs, then laughs. “Having the detector operate as designed would be an accomplishment,” he says. “It’s still a real scientific result, whether or not we see it.”
When the Nobel physics prize was announced on Oct. 6, McDonald was predictably deluged with phone calls. In a late afternoon interview with the Star, he was exhausted. But he perked up when the subject turned to DEAP.
The Sudbury Neutrino Observatory had given Canada one “eureka” moment, he said, and the DEAP team is hopeful it can provide another one.
“The big thing, though,” he added, “is that our students have the idea that they can make a difference in terms of really changing the way we look at things in a fundamental way in physics, and they can do it here in Canada.”

Expensive science
$65 million Cost of expanding SNO into SNOLAB, completed in 2011
$8 million SNOLAB’s annual operating cost
$20 million Cost of constructing DEAP, to date
10 Number of institutions collaborating on DEAP: in Canada, Carleton University, Queen’s University, TRIUMF, SNOLAB, University of Alberta and Laurentian University; in the U.K., Royal Holloway (University of London), Rutherford Appleton Laboratory, and University of Sussex; and National Autonomous University of Mexico.
SNOLAB construction funding Provided by Canadian Foundation for Innovation (CFI), Ontario Innovation trust, Northern Ontario Heritage Fund, and FedNor.
SNOLAB operational funding Provided by Ontario Research Fund’s Research Excellence Program, NSERC, CFI and member institutions. Vale provides in-kind funding. The city of Sudbury has provided a five-year grant for public education.

Source: http://www.thestar.com/news/insight/2015/12/07/in-a-sudbury-mine-physicists-probe-secrets-of-dark-matter.html