Nobel Laureate Arthur B. MacDonald on how the neutrino experiments can provide a fuller picture for the very basic laws of physics.

Nobel Laureate Arthur B. MacDonald on how the neutrino experiments can provide a fuller picture for the very basic laws of physics.

On October 6, Takaaki Kajita, Chief Scientist of Super-Kamiokande Collaboration, University of Tokyo, and Arthur B. McDonald, chief scientist at the Sudbury Neutrino Observatory (SNO) Collaboration, Queen’s University, Kingston, Canada, were awarded the Nobel Prize in Physics for proving that neutrinos change identities or ‘flavours’ from one type to another over time.

Shubashree Desikan
Professor McDonald, in this email interview with Shubashree Desikan, speaks about the 17-year journey from the start of the SNO Collaboration to the prestigious moment of recognition. Excerpts:

How does it feel to have won the Nobel Prize? Some physicists told me that it was widely anticipated that the observation of neutrino oscillations would win the Nobel this year.

Our SNO Collaboration was very pleased to have provided clear observation of neutrino oscillations for solar neutrino and to have verified models of the sun with great accuracy. However, having this strong international endorsement of the value of our work by a respected committee is very gratifying.

When you became the spokesperson for the Sudbury Neutrino Observatory in 1989, it was unusual then for Canada to make an investment of this magnitude on a basic science project. How did you convince them?

At the time that our scientists were attempting to obtain approval of funding for this project, it was necessary to convince agencies in Canada, the U.S. and the U.K. of the importance of the measurements that we could perform. It took a long time and involved technical demonstrations of the engineering feasibility of the design. With strong peer review of the importance of the basic science we were proposing (now proven by the awarding of the Nobel Prize), we were able to obtain the funding.

The return on investment is a fundamental improvement in our knowledge of our world, excellent training of the best students who are attracted to such projects, and the innovative technology that had to be developed to accomplish this difficult experiment.

“ A major question remains about the ordering of the relative masses of neutrinos, which will be addressed with sensitivity by the India-based Neutrino Observatory detector.”

— Arthur McDonald
The SNO detector consists of a 12-m diameter transparent acrylic container holding 1,000 litres of ultrapure heavy water, which was supplied by Canada’s Department of Atomic Energy. Did this happen readily? Was there any worry that you could be carrying out secret research on nuclear weapons?

Senior scientists at Atomic Energy of Canada understood the importance of the research that we would perform and recommended the loan near the beginning of our discussions. There was then a bit of worry from the public about the relationship of heavy water to nuclear reactors, etc. However, one of our scientists drank a glass of heavy water at a public meeting (with a bit of Scotch for flavour) and showed clearly that it was a safe substance. We also pointed out continually that we were creating one of the lowest radioactivity locations in the world, far from worries about high radioactivity.

What main questions remain about the neutrino and why is neutrino research important? What other experiments are being planned around the world?

A major question remains about the ordering of the relative masses of neutrinos, which will be addressed with good sensitivity by the India-based Neutrino Observatory (INO) detector. In addition, the accurate values for other oscillation parameters that INO will provide will be important for developing detailed models of how finite mass neutrinos should be added to the Standard Model of elementary particles. Other questions that will be addressed in other experiments are the absolute masses of neutrinos and the asymmetry between matter and anti-matter in the universe that may be related to neutrino properties.

Neutrino science is important because they (neutrinos), along with electrons and quarks, are the basic building blocks which we do not know how to sub-divide further. Therefore, understanding their properties is essential to complete our knowledge of the world at a very fundamental level. For example, the observation of finite neutrino masses determined by the SNO and Super-Kamiokande experiments goes beyond the Standard Model of elementary particles and may help provide a fuller picture for the very basic laws of physics.

How did the offer of the site for the observatory by INCO Ltd, the nickel mining company, come about?

Again, after presentations by our scientists, senior members of the company management saw the importance of the science that could be done and recommended to their management that we be allowed to co-exist with ongoing mining operations at the mine. This strong support for the new international underground laboratory, SNOLAB, has continued with the present owner, Vale.

Did you have to develop any new engineering techniques or materials for the construction and support of the huge underground cavity? How did you ensure the stability of the rock above the underground observatory?

It was the largest cavity of its type built at that depth, but INCO engineers provided a design of excavation and rock support techniques that were reviewed by a panel of international experts and found to be feasible. The cavity was very carefully instrumented and provided wonderful data, enabling INCO to pursue large cavities at great depth for their ongoing mining operations. Several old and new techniques were combined for ground control for the cavity, and they worked very well.

Would you say 17 years is a long time to work without an indication of whether you were on the right track?

We knew that we could have a substantial impact on fundamental physics if we could carry out this major project and that inspired everyone working on the project throughout.

Looking back on over 30 years of your association with SNO, can you tell us what made it worth the while?

Our collaboration set about understanding our universe more fully and with our results, we feel that we have made a major contribution to that. The Nobel Prize for our work is a confirmation by a body that we greatly respect that we have made a truly valuable contribution.

When we were analysing our data, we purposely added in a known amount of false data, so that those doing the analysis could not be led to a pre-conceived result by the way they analyse the data. On one day when we had defined the best way to do the analysis, we lifted this “blindness condition,” removed the false events and all together were able to see our final results. The results were conclusive that solar neutrinos did change from one type to another and, therefore, do have a finite mass. That was a real “eureka” moment that every SNO collaborator remembers as a significant day in their scientific life