Neutrinos Caught Shape-Shifting Among Three Physical Forms

A candidate electron neutrino appears in the Super Kamiokande particle detector in Japan like this. In July 2013 researchers announced they'd definitively measured muon neutrinos oscillating flavor into electron neutrinos. Credit: T2K

Exotic particles called neutrinos have been caught in the act of shape-shifting, switching from one flavor to another, in a discovery that could help solve the mystery of antimatter.

Neutrinos come in three flavors — electron, muon and tau — and have been known to change, or oscillate, between certain flavors. Now, for the first time, scientists can definitively say they've discovered muon neutrinos changing into electron neutrinos.

The discovery was made at the T2K neutrino experiment in Japan, where scientists sent a beam of muon neutrinos from the J-PARC laboratory in Tokai Village on the eastern coast of Japan, streaming 183 miles (295 km) away to the Super-Kamiokande neutrino detector in the mountains of Japan's northwest.

The Super Kamiokande neutrino detector in Japan is a cylindrical stainless steel tank that holds 50,000 tons of ultra-pure water.
Credit: Kamioka Observatory, ICRR, University of Tokyo

The researchers detected an average of 22.5 electron neutrinos in the beam that reached the Super-Kamiokande detector, suggesting a certain portion of the the muon neutrinos had oscillated into electron neutrinos; if no oscillation had occurred, the researchers should have detected just 6.4 electron neutrinos.[Wacky Physics: The Coolest Little Particles In Nature]

In 2011, T2K scientists announced they'd seen indications that this shape-shifting was taking place, but they couldn't say with certainty that the effect wasn't one of chance. The experiment has now collected enough data for the researchers to say the probability of this effect being produced by random statistical fluctuations is less than one in a trillion. The results were announced Friday (July 19) at the European Physical Society meeting in Stockholm.

The discovery opens an intriguing avenue for studying antimatter, the strange cousin of matter that's mysteriously missing in the universe. Scientists think the Big Bang produced about as much matter as antimatter, but most of this antimatter was destroyed in collisions with matter, leaving a slight excess of matter to make up the universe we see today.

The best shot at explaining why matter won out in this cosmic struggle is to find instances where a matter particle behaves differently than its antimatter counterpart. Many physicists suspect that neutrino oscillations might be just the type of occasion to see this difference.

Now that the researchers have observed this oscillation pattern in neutrinos, they can recreate the experiment with a beam of anti-muon neutrinos, and find out whether they change more or less often into anti-electron neutrinos.

"Our findings now open the possibility to study this process for neutrinos and their antimatter partners, the anti-neutrinos," physicist Alfons Weber of the U.K.'s Science and Technology Facilities Council and the University of Oxford, said in a statement. "A difference in the rate of electron or anti-electron neutrino being produced may lead us to understand why there is so much more matter than antimatter in the universe. The neutrino may be the very reason we are here."

This next phase of the project will likely take at least a decade, the researchers said.

"We have seen a new way for neutrinos to change, and now we have to find out if neutrinos and anti-neutrinos do it the same way," T2K team member Dave Wark of the Science and Technology Facilities Council said in a statement. "If they don't, it may be a clue to help solve the mystery of where the matter in the universe came from in the first place. Surely answering that is worth a couple of decades of work!"

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