Dr. Christopher Bird

The Nobel Prize committee has spoken, and the recipient for the 2015 Nobel Prize in Physics is Takaaki Kajita and Arthur B. McDonald for their discovery of neutrino oscillations! Congratulations to both the winners themselves and to their teams who made it possible.

By now the story of the neutrino oscillation is well known, at least in academic circles, and so it is fitting that the actual discovery should receive an award. 

The story begins in 1930, when physicists were puzzled at beta decays (a form of radiation) in which it appeared that energy and momentum were not being conserved. Rather than give up two sacred conservation laws, which in turn would mean that the laws of physics themselves were not constant everywhere and at every time, Wolfgang Pauli suggested that perhaps there is a particle which no one can detect. This particle would be very light, and have no electrical charge, leading to it being christened the "neutrino" or little neutral one.

In the years that followed, scientists used numerous methods to detect the neutrinos, and their existence was confirmed. In fact, experiments determined that there were at least three types of neutrino in existence, dubbed the electron neutrino, muon neutrino, and tau neutrino. But there was still an odd problem.

Physicist Ray Davis was running an experiment in which he used huge tanks of perchloroethylene (aka 100,000 gallons of drycleaning fluid) to detect neutrinos from the sun. Because the Sun generates energy through nuclear fusion, it is expected to produce neutrinos that will hit the Earth. Those neutrinos normally pass through the Earth without interacting with anything, but on rare occasions one will convert a single nuclei to some other species of nuclei. In Davis' experiment, a few chlorine nuclei become argon atoms, and these can be detected and counted. By knowing the rate at which neutrinos convert chlorine nuclei to argon, scientists could deduce the number of neutrinos generated by the Sun.

And that number was wrong. 

The best models of the Sun gave a number nearly three times higher than what the Davis experiment was measuring. And this wasn't a brief measurement either - the Homestake experiment ran from 1970 to 1994 and gave consistent numbers all along. Either the solar model were wrong, or something was destroying the normally stable neutrinos before they arrived.

The solution came on June 18, 2001 when the team at the Sudbury Neutrino Observatory in Sudbury, Ontario, Canada announced that they had detected neutrino oscillations. It was discovered that over long distance, an electron neutrino can suddenly change into a muon neutrino or a tau neutrino. And so in the distance between the Sun and the Earth, the expected flux of electron neutrinos morphed into a mixture of all three neutrino species. And since only one species could be detected at Homestake, it created this apparent discrepancy. (This result also is important in other fields of physics, because it proved that neutrinos have mass. According to the theory of relativity, a massless object does not experience time and as such cannot oscillate. Because neutrinos are now known to oscillate, they must also have mass.)

The results were later confirmed and the numbers improved by the Kamiokande experiment in Japan, and Ray Davis was awarded the Nobel Prize in 2002.

And now, thirteen years later the Nobel Prize committee has recognized the contributions of both the Sudbury Neutrino Observatory and Kamiokande by awarding the 2015 prize in physics to the respective team leaders. Congratulations to everyone involved!