And the 2015 Nobel Prize in Physics goes to….. Arthur B. McDonald and Takaaki Kajita for the discovery of neutrino oscillations, which show that neutrinos have mass. It fills me with pride to see that a Canadian scientist can win the most distinguished award for Physics in the world, and proves that cutting edge research is done by Canadian Universities. We are an important part of the global machine that is advancing humanity’s understanding of science.
So what did this East-West collaboration discover?
A long Standing Problem in particle Physics, called the ‘solar neutrino problem,’ developed back in the 1960s. Theoretical Physicists, using their understanding of the standard model, predicted the number of neutrinos we should detect from the Sun. The problem was that experiments only detected about 1/3 of the number we should expect to see in a given time period.
The Sun is like a giant nuclear reactor, where four Hydrogen Nuclei are converted into Helium, neutrinos, positrons, and energy. By understanding the interaction and using conservation of energy, we can predict how many neutrinos should be produced in a chain reaction. And by looking at the total mass of the Sun and it’s energy output, we can predict the total number of neutrinos produced and determine how many should hit Earth.
But detectors on either side of the planet could only find from 1/3 to 1/2 the number predicted. To account for the discrepancy, physicist Bruno Pontecorvo proposed that if neutrinos had mass, they could change from one of their three forms (electron neutrino, muon neutrino, tau neutrino) to another.
The experimental evidence came from McDonald in 2002 while working at the Sudbury Neutrino Observatory in Sudbury, Ontario, Canada. He devised an experimental method to detect all types of neutrinos, while distinguishing between electron neutrinos and the other two types. This showed that the correct number of neutrinos were present, they were just changing their identity on the way to Earth. This led to the conclusion that neutrinos do in fact have mass, proving that the standard model of particle physics is no the complete picture explaining the basic constituents of the universe.
Neutrinos are created in the mighty crucible of the Sun, and in the Earth’s atmosphere. Trillions of them are passing through out bodies unimpeded each second, since they rarely interact with normal matter. Yet for the vast number that exist in nature, they are the most elusive elementary particle.
Like all great scientists, this years winners share credit with their collaborators, showing that science is indeed a team sport, and everybody wins!