I am an astrophysicist and the director of the Sudbury Neutrino Observatory Institute. I hold the Gordon and Patricia Gray Chair in Particle Astrophysics at Queen's University.
Physicists have been investigating whether or not neutrinos change from one type to another. Since the late 1960s, experiments have hinted that neutrinos may do this. Theoretical models of the Sun predict that neutrinos should be made in staggering numbers. Neutrino detectors on the Earth have repeatedly seen fewer than the expected number of neutrinos. Because neutrinos come in three varieties (electron, muon, and tau neutrinos), and because solar neutrino detectors have been primarily sensitive only to electron neutrinos, one explanation over the years was that those "missing" neutrinos had changed, into a variety for which the detectors had little or no sensitivity. If true, this would require modifications to the Standard Model for elementary particles.
In August 2001, a collaboration at the Sudbury Neutrino Observatory (SNO), a detector facility located 6,800 feet (2,100 m) underground in a mine outside Sudbury, Ontario, led by myself, checked in with a direct observation showing clearly that electron neutrinos from the Sun really were changing into muon and tau neutrinos. If neutrinos do change their type in this way, the underlying theory, consistent with all experiments to date, implies that they have a mass greater than zero. SNO published its report in the August 13, 2001, issue of Physical Review Letters, and it is widely considered as a very important result.
I am honoured to be a co-recipient of the 2007 Benjamin Franklin Medal in Physics, the 2015 Nobel Prize in Physics for the discovery of neutrino oscillations and demonstrating that neutrinos have mass, and the 2016 Breakthrough Prize in Fundamental Physics.
[photo credit: University Communications]