Physicists from Japan and Canada win Nobel Prize in Physics for neutrino oscillations

Takaaki Kajita and Arthur B. McDonald, two physicists from Japan and Canada, have won the 2015 Nobel Prize in Physics for experiments that showed that neutrinos change identities, proving that neutrinos have mass and radically changing our understanding of the innermost workings of matter.

The recipients of this year's Nobel Prize in Physics were announced Tuesday at a press conference in Sweden by Göran Hansson, the Permanent Secretary of the Royal Swedish Academy of Sciences.

"A neutrino puzzle that physicists had wrestled with for decades had been resolved," the academy said in a statement announcing the recipients of the prize, which comes with a monetary award of 8 million Swedish kronor ($963,000). "The discovery led to the far-reaching conclusion that neutrinos, which for a long time were considered massless, must have some mass, however small. For particle physics this was a historic discovery."

Around the turn of the millennium, Takaaki Kajita presented the discovery that neutrinos from the atmosphere switch between two identities on their way to the Super-Kamiokande detector in Japan. At the same time, a research group in Canada led by Arthur B. McDonald was able to demonstrate that the neutrinos from the Sun were not disappearing on their way to Earth and instead were captured with a different identity when arriving to the Sudbury Neutrino Observatory.

The discovery fundamentally changed the understanding of particle physics. Its Standard Model of the innermost workings of matter had been incredibly successful, having resisted all experimental challenges for more than 20 years. But the model required neutrinos to be massless, and the observations led by Takaaki Kajita and Arthur B. McDonald had clearly shown that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.

After photons, the particles of light, neutrinos are the most numerous in the entire cosmos and constantly bombard Earth. Many neutrinos are created in reactions between cosmic radiation and the Earth’s atmosphere. Others are produced in nuclear reactions inside the Sun. Thousands of billions of neutrinos are streaming through our bodies each second and hardly anything can stop them passing, making them nature's most elusive elementary particles.

Experiments involving neutrinos are continuing worldwide and intense activity is underway in order to capture neutrinos and examine their properties. Physicists expect that future discoveries about neutrinos will change our current understanding of the history, structure and future fate of the universe.

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