2015 Nobel Prize honors discovery of neutrino oscillations!

Today, the Nobel Committee chose to recognize Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen’s University in Canada for the discovery of neutrino oscillations in the Super Kamiokande and SNO experiments respectively.  In these experiments it was proven conclusively that neutrinos can and do change their flavor, a phenomenon which requires neutrinos to have a non-zero mass.

When Ray Davis first detected the electron-type neutrinos that were predicted to come from the sun in the 1960s, the measured flux was only roughly 1/3 of what was predicted by the standard solar model.  A possible explanation of this deficit was the changing of the flavor of the some of the emitted neutrinos by a process predicted by Bruno Pontecorvo in 1957.

In 1998, the Super-Kamiokande experiment, an office-building-sized neutrino detector constructed in a zinc mine in Japan, measured the relative flavor composition of neutrinos created in cosmic rays.  It was observed that the flavor composition was different for downward-going neutrinos (those coming from the atmosphere above the detector) and upward-going neutrinos (those which were produced on the opposite side of the Earth, and had to travel through thousands of miles of rock to reach the detector).  This showed that these atmospheric neutrinos were oscillating into different flavors in flight.

With neutrino oscillations in mind as the solution to the flavor deficit seen in Ray Davis’ solar neutrino experiment, the SNO experiment was built in a nickel mine near Sudbury, Ontario, with the aim of measuring both the flux of electron-type neutrinos coming from the sun, and the total flux of neutrinos of all flavors.  The total flux measurement agreed with predictions from the standard solar model, and the same deficit in electron-type neutrinos was observed, proving that the neutrinos from the sun were changing flavor via neutrino oscillations, and definitively solving the so-called “solar neutrino problem.”

View the prize announcement here.