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dimensions of particle physics

dimensions of particle physics

A joint Fermilab/SLAC publication

 

Voyage to SNOLAB

November 05, 2012

Voyage to SNOLAB

A growing number of scientists are looking for ways to join a dream team of experiments in a unique laboratory a mile and a half underground in Ontario. There, they seek to solve some of the biggest mysteries in physics today, including the case of missing dark matter.

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In September, postdoc Hugh Lippincott prepared for a roadtrip that would take him and physicist Erik Ramberg northeast from their starting point near Chicago through Michigan and across the Canadian border. He stocked a cargo van they rented for the occasion with granola bars, apples and an iPod heavy on Pearl Jam. But this was no joyride. This was a practice run.

Loaded on springs and packed in a wooden crate in the back of the van was a stand-in for a piece of precious cargo: the quartz bell jar that makes up the heart and soul of the 60-kilogram COUPP dark matter experiment. Lippincott and Ramberg will soon transport the real thing—worth about $100,000 and a year of work—from Fermi National Accelerator Laboratory, just outside Chicago, to SNOLAB, an underground laboratory located 700 miles away in an active mine in Sudbury, Ontario.

Lippincott and Ramberg are not the first experimentalists to consider it worth the risk to move fragile equipment hundreds of miles to reach the laboratory, nor will they be the last. SNOLAB began as a single experiment looking to answer one of the biggest particle physics questions of its time. It has since expanded into an Olympic village of experiments tackling some of the most pressing questions of today.

The venue

Sudbury is the largest city in Northern Ontario. It is home to two colleges, one university, more than 300 lakes, the largest nickel-smelting operation in the world and Big Nickel, a 30-foot replica of a Canadian coin. Underneath it all lies the deepest laboratory on the continent, featuring a dedicated staff and the largest underground clean room on the planet.

The story of how SNOLAB came to be begins about 1.8 billion years ago. That’s when it is believed that a meteorite crashed in Ontario, leaving an oval scar between 20 and 40 miles in diameter that is still visible via satellite today. The impact left a crater filled with magma, which crept into cracks in the rock. The magma hardened into deep veins of precious metals and minerals and solidified the area’s future as a mining town.

In 1901, Inco Ltd. dug into those veins to build the Creighton Mine, now the deepest mine in North America.  It’s also a particle astrophysicist’s dream. That’s because scientists that study rarely interacting particles from space share a common enemy: detector-cluttering particles called background.

Scientists on experiments at SNOLAB wait for particles such as dark matter or neutrinos to bump into atoms in their detectors. When one of these tiny collisions occurs, it releases a small amount of energy that the detector picks up as a signal. On the surface, cosmic-ray particles fall in a constant barrage from space, collide with the sensitive detectors and obscure the signals physicists actually want to find. Deep underground, layers of protective rock shield experiments from this distracting extraterrestrial sleet.

First event: The solar neutrino problem

The first experiment to take advantage of the depth of Creighton Mine was the Sudbury Neutrino Observatory, or SNO, which reported its first results in 2001. SNO tackled what was then one of the biggest questions in particle physics: Where were the missing solar neutrinos?

The case of the missing neutrinos began in the late 1960s, when physicist Ray Davis Jr. collaborated with astrophysicist John Bahcall to run an experiment at another mine in Lead, South Dakota. They set out to count neutrinos let loose on the Earth by nuclear fusion in the Sun. Davis’ experiment came up with a puzzling result; he saw only one-third the number of neutrinos Bahcall’s calculations predicted they would find.

Subsequent experiments in Japan, Italy and the former Soviet Union each came down with their own cases of missing particles. For decades, the so-called “solar neutrino problem” was one of the biggest mysteries in particle physics.

But a solution was in the works. Scientists had begun to discover that there are multiple types, or flavors, of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Soviet physicist Bruno Pontecorvo, along with Japanese physicists Ziro Maki, Masami Nakagawa and Shoichi Sakata, developed the idea that neutrinos could oscillate, or change from one flavor to the next.

Perhaps the way for these experiments to capture the missing neutrinos was to expand their searches beyond a single flavor. In 1984, a group of physicists led by Herb Chen of University of California, Irvine, and George Ewan of Queen’s University, Kingston, proposed SNO, the first experiment that would be able to detect multiple flavors of neutrinos at once.

Two years later, Soviet physicists Stanislav Mikheyev and Alexei Smirnov, building on the work of American physicist Lincoln Wolfenstein, postulated that solar neutrinos might seem particularly truant because traveling through the core of the Sun amplified the oscillation process.

“The idea that neutrinos might oscillate goes back a long way,” says Carleton University physicist David Sinclair, who played a leading role in the SNO experiment and later became the first director of SNOLAB. “But before the work of Mikheyev and Smirnov, nobody took it very seriously.”

The SNO experiment had gained the momentum it needed to begin. Scientists from Canada, the United States and the United Kingdom worked with the Vale Canada Ltd. mining company (formerly Inco) to excavate a new cavern shielded from detector-muddying cosmic rays beneath 6800 feet of rock. One elevator ride at a time, over about a decade, they hauled in the components to build a 10-story, spherical detector filled with 9600 photomultiplier tubes, extremely sensitive detectors of light. It looked like a giant disco ball.

On June 18, 2001, the scientists announced their first results: SNO had solved the solar neutrino problem. The missing neutrinos were still there; they had indeed just oscillated to a different flavor.

The SNO experiment had turned theory on its head and opened doors to new realms of discovery. The next collaboration meeting turned into a party.

With newfound momentum, the physicists embarked on a larger quest: sharing their golden spot deep in the mine with other particle physics experiments. In 2002, a coalition of Canadian universities won funding from the Canadian government to expand the SNO experiment into a multi-purpose laboratory.

The Olympic team

SNOLAB’s founding experiment solved one of the biggest physics mysteries of the past. SNOLAB’s current residents hope to solve some of the biggest physics mysteries of the present.

One of those questions is: What is dark matter? All of the matter we can see is thought to make up a meager 4 percent of the universe. Invisible dark matter is thought to make up 23 percent. And yet no one has directly detected it.

“For the physics community as a whole, the identity of dark matter is a huge question,” says Michael Salamon, US Department of Energy program manager for the dark matter program.

Four SNOLAB collaborations, COUPP, DEAP, CLEAN and PICASSO, are developing new dark matter detectors, each looking for the missing matter using different technologies. COUPP will use a bubble chamber, a vessel full of liquid balanced at the precise pressure and temperature to cause it to boil if the right particle zips through; DEAP will use a detector filled with liquid argon scintillator, a substance that emits flashes of light that mark passing particles; CLEAN, similarly, will use a detector filled with liquid argon or liquid neon scintillator; and PICASSO will use a detector much like the dosimeters used to measure radiation exposure at nuclear power plants. At least two additional dark matter experiments, CDMS and DAMIC, have been assigned spaces in the laboratory as well.

The US Department of Energy and SNOLAB, along with other national agencies from around the world, support dark matter experiments that use a variety of different technologies to search for roughly the same thing. That’s important when you’re looking for something that’s never been directly observed before.

“I don’t think any single dark matter experiment is going to be able to demonstrate the existence of dark matter,” says Juan Collar, spokesperson for the COUPP experiment. “In my opinion, it will only be when a number of different experiments using different techniques start to coincide. That’s what’s going to make us convinced.”

Passing the torch

After two days of driving, one stop at Port Huron customs center and one stomach-dropping snapping sound from the back of the van—it turned out to be nothing more than a strap settling—Lippincott and Ramberg arrived in Sudbury with the prototype inner vessel intact.

The next morning, they rose before dawn to catch the 6 a.m. elevator down to SNOLAB. Physicists have one, maybe two chances each morning to enter the mine. Similarly, they have only a couple of chances to go back to the surface at the end of the day, around 4 p.m.

They changed into mining coveralls and heavy boots and carefully unloaded the 200-pound prototype detector into a mining cart, which they pushed into the metal cage of the mine elevator.

The rattling elevator can drop the mile and a half to SNOLAB in just over three minutes. “When you’re going down, you feel this wind,” Lippincott says. “Your ears pop.”

From there, the road to SNOLAB is more than a mile of lamp-lit tunnel, a humid passage called a “drift” in mining terminology. The ambient rock temperature at this depth is 107 degrees Fahrenheit, but air ventilation keeps it at about 80 degrees. Travelers walk in single file toward the bright white light of the laboratory. “You can see the dust floating through the headlamps,” Lippincott says.

SNOLAB is kept at 72 degrees Fahrenheit and at a higher pressure than the air around it to repel dust. As Lippincott and Ramberg approached the entrance, they felt the flow of air down the tunnel and a light spray of dirt-defeating mist.

New arrivals cannot just step through the boundary between the hot, grimy world of the mine and the cool, ultraclean world of the laboratory. Lippincott and Ramberg needed to shower first. Afterward, they changed into special clothing and hairnets kept and even laundered below ground. Their cargo also needed a rinse; the prototype inner vessel passed through an area called “the car wash.”

Inside SNOLAB, “it’s much bigger than you would think it would be,” Lippincott says. “It’s echoey in the big halls. There are bright lights. Everything is white-washed.” Air chillers and filters hum in the background.

They had finally reached their destination, and the test cargo had survived the trip. It had been an arduous journey, but members of the COUPP collaboration think it’s worth the trouble to conduct their experiment in this place. As early as this month, Lippincott and Ramberg will do it all again, this time with the real detector. Next year, they should have their first results from the new detector’s search for dark matter.

 

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Illustration by Sandbox Studio, Chicago