Neutrino shape-shift points to new physics

Neutrinos have been caught shape-shifting in a way no one has seen before, a particle experiment has confirmed. Delayed for years by the magnitude-9 earthquake that rocked Japan in March 2011, the result is an early step towards figuring out why there is more matter than antimatter in the universe, and possibly opens the door to new physics.

Notoriously shifty particles, neutrinos are nearly massless and scarcely interact with other particles, slipping like ghosts through kilometres of rock. They are also masters of disguise that come in three flavours ? electron, muon and tau ? and are believed to be able to flip-flop between these types without warning in a process called oscillation.

The problem is, most experiments measure the rate of neutrino oscillation by starting with one neutrino type and seeing how many of them disappear by the time the particles reach a detector, rather than seeing the transformed neutrino arrive anywhere. For example, until 2011, no one had seen any solid signs of muon neutrinos turning into electron neutrinos.

The T2K experiment in Japan generates a beam of muon neutrinos at the J-PARC accelerator in Tokai, near Japan's east coast. It sends them 295 kilometres to the Super-Kamiokande neutrino detector in Kamioka, on the west coast. In 2011 the team saw the first hint of the transformation, but the 2011 megaquake temporarily shut down the experiment before it could confirm the sighting.

Antimatter mystery

Now, with about four times as much data, T2K is finally able to claim certainty. They have detected a total of 28 electron neutrinos, when fewer than 5 would be expected if the neutrinos were not oscillating. Odds that the result is a fluke are less than one in a trillion. The team announced the results today at the European Physical Society meeting in Stockholm, Sweden.

Previously, this type of neutrino oscillation was only indicated, says David Wark of the Science and Technology Facilities Council in the UK, and a member of the T2K collaboration. "Now it counts as a discovery."

The result offers a path towards solving one of the biggest mysteries in physics: why there is more matter than antimatter in the universe. Standard theories say that matter and antimatter should have been created in equal amounts by the big bang. But for some reason, matter won out.

Particle surprise

Now that we have seen the muon neutrino morph into the electron neutrino in normal matter, physicists can run the T2K experiment with a beam of anti-muon neutrinos. Subtle differences in the way neutrinos and antineutrinos oscillate could have skewed the ratios of matter and antimatter production in the early universe. "This is the first step along the way, but it proves that we're going to get there," says Wark.

The measurement is also interesting in its own right, says Janet Conrad of the Massachusetts Institute of Technology, and a member of the Double Chooz neutrino experiment in Chooz, France. Since 2011 that detector and other "disappearance" experiments have seen indirect signs of the muon-electron shift.

Comparing T2K's results to the disappearance data can directly point the way to new laws of physics beyond our current understanding. "If we see inconsistencies, it means there's new physics going on," says Conrad. "Neutrinos always surprise us, so this is an exciting opportunity to see what more these particles may have to say."

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