Ryan Wilkinson, Durham University and Celine Boehm, Durham University
Canada’s Arthur B McDonald and Japan’s Takaaki Kajita have won this year’s
Nobel Prize in Physics for their surprising discovery that tiny, subatomic particles called neutrinos have mass.
Their experimental results forced scientists to rethink the “Standard Model” of particle physics that had successfully explained all observations of the subatomic world for decades.
What are neutrinos?
Neutrinos are produced when radioactive isotopes decay and have been shrouded in mystery ever since Wolfgang Pauli first proposed them in 1930. In the Standard Model, they were assumed to have no mass (like particles of light, photons) and be neutral (lacking electric charge). This would also explain why neutrinos usually pass straight through matter without interacting, making them extremely difficult to detect. Enormous instruments are required to observe them in sufficient numbers to study their properties.
Neutrinos were first directly observed by the Cowan-Reines experiment in 1956, using neutrinos from a nuclear reactor and two large tanks of water. If a neutrino interacted with a nucleus in the detector, this would result in a flash of light that could be picked up by photomultiplier tubes that were sandwiched between the tanks. Frederick Reines was awarded the Nobel Prize in 1995 for this work.

MissMJ – Own work by uploader, PBS NOVA, Fermilab, Office of Science, United States Department of Energy, Particle Data Group, CC BY
However, when detectors became sensitive enough to observe neutrinos created in nuclear reactions in the Sun, scientists faced a big problem.
They had calculated the amount of neutrinos from the Sun that should be hitting the Earth, but observed only a third of this number in their experiments. A further Nobel Prize was presented to Ray Davis in 2002 for this discovery. The mystery of these missing neutrinos was coined the “solar neutrino problem” and remained a puzzle for forty years, until the collaborations led by Kajita and McDonald made their exciting discovery.
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