The 2015 Nobel Prize in physics goes to Takaaki Kajita and Arthur B. McDonald for their work on neutrino oscillations. By tracking neutrinos in subterranean water tanks, the researchers watched neutrinos change flavour, in turn proving that the subatomic particles have mass.

Two research groups—the Super-Kamiokande led by Takaaki Kajita in Japan and the Sudbury Neutrino Observatory led by Arthur B. McDonald in Canada—each contributed to the discovery of neutrino oscillation. Neutrino oscillation is the process by which neutrinos switch between the three primary flavours: electron neutrinos, mu neutrinos and tau neutrinos. As a consequence of oscillation, neutrinos must have mass which breaks from the original Standard Model theory of particle physics.

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As a neutrino travels through space, quantum mechanical fluctuations change the particle through the different flavours of neutrino in a periodic cycle, mixing electron, mu, and tao states. The Super-Kamiokande observed atmospheric neutrino oscillation from mega to tera electronvolts (MeV to TeV), tracking the changes. Meanwhile, the Sudbury Neutrino Observatory recorded solar neutrinos flux, counting each flavour, noticing how they changed and proving that they weren’t just vanishing somewhere within the planet. Both observatories are massive subterranean tanks of water, sheltered from more casual particle interactions.

The Sudbury Neutrino Observatory. Courtesy: Lawrence Berkeley National Laboratory

The press release for the Nobel Prize announcement reads:

The Nobel Prize in Physics 2015 recognises Takaaki Kajita in Japan and Arthur B. McDonald in Canada, for their key contributions to the experiments which demonstrated that neutrinos change identities. This metamorphosis requires that neutrinos have mass. The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe.

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.

Meanwhile, the research group in Canada led by Arthur B. McDonald could demonstrate that the neutrinos from the Sun were not disappearing on their way to Earth. Instead they were captured with a different identity when arriving to the Sudbury Neutrino Observatory.

A neutrino puzzle that physicists had wrestled with for decades had been resolved. Compared to theoretical calculations of the number of neutrinos, up to two thirds of the neutrinos were missing in measurements performed on Earth. Now, the two experiments discovered that the neutrinos had changed identities.

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. Its Standard Model of the innermost workings of matter had been incredibly successful, having resisted all experimental challenges for more than twenty years. However, as it requires neutrinos to be massless, the new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.

The discovery rewarded with this year’s Nobel Prize in Physics have yielded crucial insights into the all but hidden world of neutrinos. After photons, the particles of light, neutrinos are the most numerous in the entire cosmos. The Earth is constantly bombarded by them.

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. Hardly anything can stop them passing; neutrinos are nature’s most elusive elementary particles.

Now the experiments continue and intense activity is underway worldwide in order to capture neutrinos and examine their properties. New discoveries about their deepest secrets are expected to change our current understanding of the history, structure and future fate of the universe.

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The Nobel Prize in medicine is going to William C. Campbell, Satoshi Ōmura, and Youyou Tu for their work on fighting parasites. The Nobel Prize for Chemistry will be announced on Wednesday, Literature on Thursday, and the Peace Prize on Friday. The award ceremony will be on December 10, 2015.

We’ll have a more in-depth explainer on neutrino oscillation later today. In the meantime, here’s our explainer from when we first learned of the phenomena.

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[Nobel Prize]

Top image: The Super-Kamiokande observatory. Courtesy: Super-Kamiokande


Contact the author at mika.mckinnon@io9.com or follow her at @MikaMcKinnon.

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