Art McDonald, Neutrinos and the Nobel Prize
At 5 am on Tuesday, Oct. 6, 2015, Arthur McDonald was woken by a phone call. The caller brought the Emeritus Professor of Physics some startling news from Stockholm: the Royal Swedish Academy had awarded him the Nobel Prize for Physics “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”
For Professor McDonald and a team of dedicated co-researchers, the news marked the happy culmination of three decades of inquiry into one of the fundamental mysteries of physics: how the substance of the universe was formed out of “dark matter.” The Nobel elation spread well beyond the Queen’s campus. In the words of Antonio Ereditato, director of Switzerland’s Albert Einstein Centre for Fundamental Physics: “This is really one of the milestones in our understanding of nature.”
Amid the rejoicing came the questions: what are neutrinos and what role do they play in the universe?
Neutrinos are sub-atomic particles radiated from the sun that travel through space and saturate the earth. Scientists have long pondered whether they were in fact building planetary structure or whether they simply dissipated. The problem was that there was no way to measure neutrinos as they entered earth’s atmosphere.
In the mix of compromising elements such as radioactivity, measuring the fate of neutrinos became impossible and their function in the creation of matter problematic. This quandary had long perplexed physicists. Traditionally, they had employed expensive, high-energy accelerators to try to isolate neutrinos, but were never able to completely separate neutrinos from their atmospheric “background.”
Building a lab deep below the earth
In the early 1980s, scientists speculated that neutrinos might be cleanly measured if the process was conducted deep underground, once neutrinos had left behind the contamination of the earth’s surface. Attention turned to the Creighton nickel mine outside Sudbury, Ontario. Here was a cavity in the earth two kilometres deep, large enough to accommodate what would be a bulky, orb-like detection system full of 1,000 tons of heavy water encased by thousands of photomultiplier tubes.
The mine owners proved accommodating and plans were put in motion. It took five years to assemble a consortium of Canadian and American universities and to convince Canadian and international granting agencies to fund the estimated $70-million project. After much vetting by international panels of experts, the Sudbury Neutrino Observatory (SNO) proposal won Canadian, American and British funding approval in 1990 and the arduous process of building the intricate and delicate detector began. Because of the narrowness of the mine shaft, the detector had to be broken down into 130 sections.
From the outset, Queen’s physicists had taken the administrative lead in the SNO project. In 1989, Professor McDonald, a PhD in nuclear physics, was appointed the first director of the project. A Nova Scotian by birth, his career had taken him to the Chalk River research labs of Atomic Energy of Canada and then to Princeton University. From the outset, his collegial personality and ability to manage the unprecedented construction and then operation of so innovative a project stood out. A massive undertaking, SNO required collaboration from a number of partner universities. Many researchers, such as Doug Hallman, George Ewan, Hay Boon Mak and Peter Skensved, to name just a few, took part in bringing the project to life.
The experiments begin
In 1998, SNO was ready. Queen’s principal Bill Leggett cut the ribbon. Special guests such as Cambridge physicist Stephen Hawking looked on approvingly. By then the SNO consortium had grown to 15 universities and research agencies. The giant deflector, studded with its photocells, looked like something out of Star Wars. The experiments began.
The observatory worked as planned: it isolated neutrinos in flux and, in what McDonald called his “eureka moment,” proved that on their journey to earth they changed their shape and therefore their finite mass. They could therefore be considered building blocks of the universe. Across the globe at the University of Tokyo, Professor Takaaki Kajita was using the Super Kamiokande detector to arrive at the same earth-shattering conclusion.
In his speech at the Stockholm award ceremony (one he shared with Professor Kajita), Art McDonald talked about the “passion” that the SNOLAB team brought to their research. Discovering the real nature of neutrinos revealed what “people with the courage to try” might achieve. Three weeks later, he got more good news when he learned that he’d won the $3-million Breakthrough Prize in Fundamental Physics for his leadership and scientific insight.
Pictured at top: Arthur McDonald photo by Pi Frisk Copyright Nobel Media