The neutrino breakthrough

In the summer of 1992, Art McDonald, a physicist and professor in the Queen's University Department of Physics, Engineering Physics & Astronomy and future Nobel laureate, had a rare encounter with a living legend.

The setting was the Canada Pavilion at Expo’92 in Seville, Spain. The encounter was with Bruno ­Pontecorvo, an Italian-born physicist who had worked under Enrico Fermi and then emigrated to Canada during the Second World War. In Canada, Dr. Pontecorvo had helped to design the Chalk River nuclear reactor. More importantly, he had done remarkably prescient work on the mysterious subatomic particle that Dr. Fermi had once dubbed “the little neutral one” – the neutrino.

Dr. McDonald was well aware of Dr. ­Pontecorvo’s resumé. By then, the neutrino had ­become the focus of his own career and lay at the heart of one of the most ambitious science projects ever attempted in Canada. The two had never met before. Dr. Pontecorvo had left Canada many years earlier and then notoriously ­defected to the Soviet Union. For western physicists, he had become a remote and enigmatic genius.

They were both in Spain to attend a conference. By then, Dr. Pontecorvo was 79 and suffering from advanced Parkinson’s. But he was anxious to see an exhibit in Seville promoting the Sudbury Neutrino Observatory (SNO), the recently approved project that Art McDonald was leading. Dr.McDonald ­offered to give Dr. Pontecorvo a personal tour.

Dr. Art McDonald, professor emeritus at Queen's University, is the co-winner of the 2015 Nobel Prize in Physics – along with Takaaki Kajita of the University of Tokyo – for groundbreaking research on neutrinos. (Photo by Alexander Mahmoud, Nobel Media)

Decades earlier, in Chalk River, Dr. Pontecorvo and physicist Geoff Hanna had performed an ­experiment that put an upper limit on the mass of the neutrino. Now SNO was setting out to discover if neutrinos had any mass at all, a key theoretical question that was linked to the nature of matter and the structure of the universe.

Dr. McDonald had also worked at Chalk River, arriving a quarter century after Dr. Pontecorvo left. He later moved to Princeton University and finally to Queen’s, in 1989, to take over the reins of the SNO project from George Ewan, one of SNO’s founders. As they toured the Seville exhibit, the real experiment was already under construction deep below ground in an Ontario nickel mine. Dr. Pontecorvo, who had written to the Canadian government in support of SNO, was enthusiastic about its prospects.

The meeting was a study in contrasts, and not just geopolitical. Dr. Pontecorvo had helped to lay the foundations for nuclear and particle physics. Dr. McDonald, then in his late 40s, was part of the next wave. And where Dr. Pontecorvo had spent most of his life working with one or two ­collaborators, Dr. McDonald was leading an ­international army of physicists and engineers, all bent on creating an experiment of epic scale in a ­subterranean warren.

Strolling in the deep

My own first encounter with SNO came in 1998 when I travelled to Sudbury to report on the ­facility – then nearing completion – for a magazine story. I knew the location of the experiment was no coincidence. Nuclear reactors in Canada ­required the use of heavy water, an expensive ­resource that was also known to be a particularly effective medium for sensing passing neutrinos. Sudbury’s Creighton Mine, operated by Inco, now Vale Canada, Limited, was one of the deepest in the country and the ideal place for the SNO team to build its giant, heavy water neutrino detector.

The journey to SNO felt like a mythological ­passage through the underworld. It began by dressing up in full miner attire: helmets with headlamps, heavy boots and overalls. My notebook and recorder were double-bagged in plastic. I would later find out why. Next, my physicist guides and I proceeded to the mine’s entrance, and a large double-decker, open air lift that would plunge us down a shaft more than two kilometres deep. As the Precambrian rock raced past us in a blur, I remember marvelling at the length of the stretched out elevator cable – long enough so that it rebounded several inches each time we reached a new level and another group of miners stepped off. But we were going all the way to the bottom.

Neutrino hunting began at street level in the 1950s when researchers learned they could detect the elusive particles as they streamed out of nuclear reactors. But at SNO, the objective was to ­observe neutrinos streaming away from the core of the sun. Since neutrinos interact so infrequently with other types of matter, they easily pass through solid rock. The advantage of going so deep was to screen out cosmic rays – high-energy particles from space that would otherwise overwhelm a sensitive neutrino detector the way a jet engine can overwhelm a quiet conversation.

I stuck close to my guides as we switched on our lamps and made our way through a broad, dark ­tunnel with occasional side passages veering off into darkness. The walk would take us about as far into the mine as the elevator had taken us down. It was surprisingly warm; without constant ventilation, Earth’s natural heat can make it unbearable to work at such a depth. ­Finally, we rounded a bend and saw the brightly lit entrance to SNO. My first ­impression was that we had come across an underground car wash.

With two kilometres of rock above it, SNO was well shielded from cosmic rays, but natural radioactivity from the surrounding rock and dust presented a formidable challenge to the sensitive experiment. To have a hope of success, SNO had to be better than operating room clean in the midst of one of the world’s dirtiest places: a working mine. To ­enter, we had to be clean, too.

We started by hosing down our boots and then shedding our mining gear. In the next room we got rid of the rest of our clothing too, then showered down – a strange sensation when one is two ­kilometres underground. Stepping from the shower to the next stage we received new ­clothing, for use only in the lab. As a final step, I passed through an air drier that was designed to blast off specks of rock dust that might still be clinging to me. I had arrived.

From mystery to history

For the novice, getting a grip on the ephemeral ­nature of neutrinos can often seem like a task better suited to Lewis Carroll’s White Queen, who claimed she could believe six impossible things before breakfast. It’s an especially fitting boast. In the ­Standard Model of particle physics – the theory that ­describes the fundamental building blocks of matter – there are six different types of neutrinos, which belong in three families or “flavours” of particles.

The first family includes the electron neutrino – so named because it is often produced in nuclear reactions that involve electrons. The electron ­neutrino comes with an anti-matter counterpart, or anti-neutrino. (In the looking-glass world of ­quantum physics, particles and anti-particles are ­regarded as mirror reflections of the same entity, If one exists, so must the other.)

The Standard Model includes two other neutrino flavours, named after the muon and the tau particle. Together with their anti-matter counterparts, muon and tau neutrinos complete the set. Dr. Pontecorvo had once helped to show that neutrinos are lighter than any other form of matter. In the simplest version of the Standard Model, neutrinos do not have mass at all. But while this makes for a tidy theory, Dr. McDonald’s team at SNO would ultimately show that’s not how nature rolls.

SNO was built because earlier efforts to detect neutrinos from the sun had repeatedly come up short. For John Bahcall, the American theorist who first wrestled with the problem in the 1960s, it was a disconcerting state of affairs. Dr. Bahcall used his understanding of the physics of the sun to calculate the expected rate of solar neutrinos reaching Earth. When his answer disagreed with experiments, he worried that his calculations were wrong. But no one could see a mistake, and year after year the deficit persisted. Either the physics of the sun was wrong or the Standard Model was. Either possibility carried huge implications for ­scientists’ understanding of the universe.

It was Bruno Pontecorvo who first proposed a solution in 1969. Like John Bahcall, he realized that the nuclear reactions that take place in the sun can only produce electron neutrinos. Dr. ­Pontecorvo reasoned that some of them were switching flavour en route to Earth and thereby escaping detection. It was a strange idea, but mathematically possible, according to the weird rules of quantum physics [see Mixing Metaphors – What SNO Discovered], provided that neutrinos have mass.

This idea set the stage for SNO. What the ­situation called for was an experiment that was sensitive to more than one flavour of neutrino. SNO was designed to do precisely that. In theory, it would be able to count up electron neutrinos interacting with the heavy water through one kind of reaction while also monitoring a second reaction that picked up neutrinos of all three flavours. If the second result gave a higher number than the first, the neutrinos from the sun were likely switching flavours.

The idea was simple but putting it into practice was not. Again and again, Dr. ­McDonald and his international team would have to rise to meet the technical hurdles and deadlines, keeping construction on track and bringing the experiment to the level of sensitivity it needed to succeed.

During my first visit to SNO, the sense of mission was apparent. My timing was ­fortunate. At that point, Dr. McDonald and his team had just finished loading the experiment’s giant acrylic vessel with 1,000 tonnes of heavy water. Before it was sealed, I was able to stand on the platform and look down the vessel’s long neck to the fluid below. I immediately understood I was looking at history in the making.

Two and a half years later, in May 2001, Dr. McDonald and the SNO team announced the result that would eventually earn him the Nobel Prize: solar neutrinos do change flavour, which means that ­neutrinos do have mass. Before revealing what SNO had found, Dr. McDonald called John Bahcall to share the news, along with Hans Bethe, the physicist who had first worked out the nuclear process by which the sun shines.

Bruno Pontecorvo, who died the year ­after he met Art McDonald in Seville, would never learn the outcome of the ­experiment. But his contributions are ­woven into the ­history of the field, in the way that every ­insight and discovery in physics serves to sow the seeds for what comes next.

The SNO experiment has already done the same. Neutrino physics has moved ­forward into the 21st century, just as SNO has grown into SNOLAB, a much larger, ­multi-experiment facility developed by a consortium of universities. Like a tunnel in the dark, it’s not yet clear where it will all lead. But thanks to Art McDonald and the many others who worked on SNO, it’s a ­journey Canada will remain part of for years to come.

The SNO Timeline