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Particles and interactions

Particles and interactions

T2K discovery puts neutrino oscillation beyond doubt

19 Jul 2013 Hamish Johnston
The SuperKamiokande detector lies 1 km underground in the Mozumi mine in the city of Hida. (Courtesy: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

By Hamish Johnston

Physicists working on the Tokai to Kamiokande (T2K) experiment have confirmed what many have suspected for nearly three decades – over time, a neutrino of one flavour will change into a neutrino of another flavour in a process called neutrino oscillation.

The team did this by firing a beam of muon neutrinos 295 km through the ground under Japan to the SuperKamiokande experiment. There, they detected the presence of electron neutrinos in the beam.

Several experiments have already shown that the number of neutrinos of a certain flavour in a beam decreases as it travels a long distance. However, this is the first time that the appearance of a different flavour in a beam has been seen with a statistical significance greater than 5σ – the gold standard for a discovery in particle physics.

The team fire the muon-neutrino beam from the J-PARC lab in Tokai. The beam passes through two detectors – the first is a “near detector” a few hundred metres from the source that characterizes the muon-neutrino flux and the second is SuperKamiokande. After running the experiment for two years, Super-Kamiokande registered 22 electron neutrinos whereas only 6 should have been detected if neutrino oscillation was not occurring. The result has a statistical significance of 7.5σ.

The discovery was presented today at the High Energy Physics 2013 conference in Stockholm by Michael Wilking of TRIUMF. His talk is entitled “The latest results from T2K on the neutrino oscillation and interactions“.

Despite this breakthrough, there is much more work to be done unravelling the mysteries of the neutrino. You can find out much more about neutrinos in the Physics World feature-length article “Neutrinos: ghosts of matter” by Dave Wark of Oxford University.

T2K is an international collaboration and Wark leads the UK contingent. “It’s a joy to see T2K deliver the science we designed it for,” says Wark. “I have been working on this for more than a decade, and what these results tell us is that we have more than another decade of work ahead of us.”

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