Office of the Vice-Principal (Research)

Office of the Vice-Principal (Research)
Office of the Vice-Principal (Research)

Big Science for Small Particles

by LOWELL COCHRANE

Photo of Dr. Tony Noble holding a piece of equipmentWhen Dr. Tony Noble describes the work going on at SNOLAB, one of Canada’s foremost facilities for astroparticle physics research, one gets an unsettling sense of humanity’s profound ignorance. But that sense is also exhilarating. “When we look into the night sky, we only see about five percent of what’s going on,” says Noble. “Even with the most powerful telescopes we can imagine, we have no way to see 80 percent of the matter that makes up our universe. We just don’t know what the other stuff is.”

That ignorance has a name: physicists call it dark matter, and it’s one of science’s biggest puzzles. It’s exactly the kind of giant puzzle that piqued Noble’s interest when he first decided to pursue particle physics as an undergraduate student, and which he continues to pursue today as SNOLAB Institute’s Director. In that role, he deals not only with questions and answers in physics, but also with a global network of people, resources and institutions that are pushing the frontiers of knowledge. 

SNOLAB is located in an unlikely spot – deep underground in Vale’s Creighton Mine, in Sudbury, Ontario. It may be one of the most cosmologically significant places on Earth. To understand why, you have to step back and picture our planet hurtling through space at well over 100,000 kilometers per hour (relative to the sun), and being continually bombarded by cosmic radiation. This makes the Earth’s surface, where we live and conduct most science experiments, a cosmologically noisy place. The omnipresent cosmic radiation makes it difficult or impossible to conduct certain kinds of sensitive experiments. Yet deep in the hard rock of the Canadian Shield, protected from that incoming bombardment from space, those same experiments become possible.

Visiting SNOLAB is a commitment. It involves taking a cage lift two kilometres below surface into a working nickel mine, and then traveling nearly two more kilometers horizontally through the mine drifts before you arrive at SNOLAB’s ultra-clean underground facility.

While a modern mine is relatively clean as far as mines go, it is nowhere near clean enough for SNOLAB’s sensitive experiments. All rock emits a certain amount of radiation, so SNOLAB has to be absolutely sure that rock dust from the mine doesn’t contaminate the facility. Everyone entering must undergo a cleansing process that includes taking a shower and exchanging their own clothes for specialized lab clothing – the spotless white kind you see in images of microchip factories. Every piece of equipment brought in undergoes a similar “car wash” process. Thanks to these stringent protocols and technologies, the lab facilities housing the sensitive experiments contain about 100 million times less radiation than what you and I experience above ground. At these levels, SNOLAB is the place to be for anyone searching for the universe’s most elusive particles.

An illustrious forebear

SNOLAB evolved out of the Sudbury Neutrino Observatory (SNO), a $60-million experiment created in the 1990s by a consortium of Canadian, US and UK institutions, including Queen’s. Its purpose was to solve a decades-old scientific mystery called “the solar neutrino problem.”

Neutrinos are tiny particles produced during fusion reactions in the sun. They stream out of stars by the trillions, filling space. They pass through all matter, even our planet, but rarely interact with it. To illustrate their profusion, Noble holds up his thumb for a second. “A billion neutrinos just passed through my thumbnail,” he says. “If you had a lead brick a light year across, you’d be lucky to stop half of the neutrinos passing through.”

The standard model of particle physics posits three different types of neutrinos, but the sun’s fusion reactions are supposed to create only one type – the electron neutrino.  All solar neutrino detectors prior to SNO were designed to detect only electron neutrinos, but scientists were mystified because they were only seeing about a third of the neutrinos that, according to their models, should have been streaming from the sun. Where were the missing neutrinos?

There was a suspicion among scientists that once solar neutrinos had left the sun they switched from one type, or flavour, to another. This phenomenon was explainable by quantum mechanics, but only if neutrinos possessed mass. It had always been assumed that neutrinos lacked mass, but no experiment had proved it conclusively. To solve this mystery once and for all, physicists had to build a detector that could observe all three flavours of neutrino. That detector was SNO.

Because neutrinos are exceptionally “shy” particles, to detect them in any quantity you need an excellent trap. SNO was designed to be exactly that. It consisted of a giant acrylic ball filled with more than 1,000 tonnes of heavy water, suspended in a rock cavern two kilometres underground and surrounded by thousands of photomultipliers – like the eye of a giant fly turned inwards on itself, staring at the heavy water for any hint of cosmic action. Once in a rare while, neutrinos from the sun would interact with a heavy-water molecule and cause a tiny flash of energy that the photomultipliers could detect. The heavy-water interactions allowed physicists to distinguish between electron neutrinos and other types, and thereby shed light on the solar neutrino mystery.

The SNO team, led by Queen’s professor, Art McDonald, published their first results in 2001. They demonstrated that neutrinos do indeed have a barely detectable mass and that they do indeed oscillate between flavours once they leave the sun. Judging by the number of awards received by SNO team members and how often their results were cited in other scientific papers, the experiment was a major Canadian success story. The new understanding of neutrinos influenced fields from nuclear physics and cosmology to particle physics.

An ambitious expansion

Image of a tunnel with a person at the end of itBut answers in any field of knowledge always suggest new questions – which is precisely how SNO evolved into SNOLAB.

“The success of SNO led to the desire to continue in this line of research looking at new problems, like searching for dark matter, looking at supernova neutrinos and learning a lot more about neutrinos themselves,” says Noble. “You have to do those experiments in an underground environment where you’re shielded from the cosmic rays.”

Scientists quickly realized that expanding SNO’s unique underground infrastructure offered a unique opportunity to create one of the globe’s premiere astro-particle physics labs. “We thought we could build a modest-sized lab to support seven or eight different experiments, and get it up and running quickly,” recalls Noble. “We could do important science of great interest to our community, and keep it at a modest size and price.”

SNO had made Canada a leader in the field of astro-particle physics, but it had to move quickly if it was to maintain that position. While a number of big science projects are in development around the globe – including at least one large underground facility – their sheer size, expense and complexity makes them risky endeavors that can take many years to realize. On the other hand, the consortium of universities that proposed SNOLAB – including Carleton University, Laurentian University, Queen’s University, the University of British Columbia, the University of Guelph and the Université de Montréal – embarked on their project with a clear, precise vision of what they wanted and how to go about making it happen in a comparatively short period of time.

Luck was on their side: a funding program established by the Canada Foundation for Innovation was announced in 2002 at roughly the same time the partners began discussing SNOLAB. They applied for the funding in 2003, were successful, and began the engineering design the following year. SNOLAB’s first experiments were up and running by 2008. Noble still marvels at how fast everything came together. “I mean, it just went bang, bang, bang because of the scale, and because the atmosphere in Canada was incredibly supportive towards it.”

Today, SNOLAB is operated through the SNOLAB Institute Board of Management, whose member institutions are Carleton University, Laurentian University, Queen’s University, the University of Alberta and the Université de Montréal. SNOLAB consists of 5000 m2 of underground laboratory and another 3000 m2 at the surface. Unlike SNO, the more recent venture isn’t a specific experiment, but a facility that supports multiple simultaneous experiments. Many of the current and future experiments at SNOLAB will hunt for evidence of particles that fit the characteristics of dark matter (see sidebar).

One such experiment, dubbed DEAP, is led by Noble’s colleague at Queen’s, Dr. Mark Boulay. It is currently running prototype experiments that will hopefully result in a new detector that will increase the sensitivity to dark matter by a factor of a thousand.

That makes sense, because SNOLAB is all about sensitivity. Developing a way to filter out every hint of background noise is key to listening for whispers of dark matter. Sometimes, that can be literal: in Noble’s other role as a SNOLAB experimentalist, he leads the PICASSO project, which detects extremely faint sound waves created when particles passing through the detector burst droplets of superheated liquid.

Other SNOLAB experiments are detecting supernova neutrinos and expanding knowledge of solar neutrinos. An experiment called SNO+, led by Dr. Mark Chen, also of Queen’s, involves filling the original SNO vessel – the one that originally contained the heavy water – with a scintillation fluid to detect all kinds of neutrinos at low energy, including neutrinos originating from Earth. The experiment will be able to fine-tune sensitivity to neutrinos to look more closely at their flavour-oscillating behaviour.

Although SNOLAB was conceived and built by astrophysicists, not every experiment there involves particle astrophysics. Ideas and proposals come from across the Canadian and international science community, and SNOLAB supports and helps actualize that research.

Experiments planned and already conducted include research on extremophile biology and seismology. There is considerable interest from the mining and geology sectors in using neutrinos to probe deep structures in the earth, such as ore bodies.

This is how SNOLAB repays society’s investment in fundamental science. This process of “payback” is not new: Noble notes that historically, most, if not all, major technological advances start with breakthroughs in our understanding of nature. SNOLAB’s goal isn’t to directly develop these technologies – but it’s almost certain that, sooner or later, new and life-changing applications will emerge from what SNOLAB scientists are learning about the invisible constituents of the universe.

Noble motions vaguely towards his office window. “Consider all the things we can see out there – all the matter. We know a lot about what that stuff is, and how it interacts. And we have laws and models to describe it in great detail. But that’s only about 20 percent of the matter that we detect in the universe. Experiments at SNOLAB are attempting to get a glimpse of this vast, unseen world that we know very little about.”

Illuminating Dark Matter

Dark matter is called dark matter because it doesn’t produce light. To be clear: it’s not matter that is just too dim to see with telescopes, but a type of matter that behaves differently than the matter we’re more familiar with. However, like ordinary matter, it interacts gravitationally, which is what betrays its existence.

Evidence for dark matter was first observed early in the 20th century, when astronomers calculated that stars at the edges of a galaxy were moving about as fast as the stars near the centre. According to traditional physics, that shouldn’t happen. Noble explains: “Any kid knows that if you sit at the outside of a playground roundabout, you’re likely to be flung off if it’s going too fast – you really have to hold on.”

Normally, gravitational strength decreases the farther you move from the centre of a mass. Logically, then, stars at the outer edges of a galaxy should move more slowly than stars near the centre just to avoid being flung out into space. But they aren’t moving slower. So either the theory of gravity is flawed, or there is far more mass in galaxies than is measurable from just the visible stuff. In fact, the speed of those outer stars suggests there should be nearly 10 times more mass than we can see. It’s as though galaxies are sitting in the middle of a giant cloud of matter much much bigger than the visible galaxy itself.

There’s other evidence of dark matter, including observations of distant galaxies that appear to terrestrial observers to be visually “smeared.” They look that way because, like objects in a funhouse mirror, the light from those distant galaxies is bending as it passes through clumps of dark matter lying between the galaxy and Earth. This light-bending phenomenon is called gravitational lensing, and is one of many hints that there’s much more happening in the darkness between the stars than first imagined.