Neutrinos produced by a nuclear reactor in China are changing from one flavour to another more rapidly than expected. The result means physicists could soon explain why the universe is filled with matter instead of featureless radiation.

　　Neutrinos and antineutrinos each come in three flavours: electron, muon and tau. As they fly through space, these particles can morph from one flavour into another.

　　This shape-shifting ability is measured by three parameters, also called mixing angles: theta12, theta23 and theta13. Until recently, only the first two mixing angles had been measured. Then, in June last year, the T2K experiment in Japan detected muon neutrinos turning into electron neutrinos, providing preliminary estimates for theta13.

　　But the T2K observations depend on other mixing angles. "So it was hard to pinpoint a unique value for theta13," says Kam-Biu Luk at the University of California, Berkeley. Now researchers at the Daya Bay Reactor Neutrino Experiment, based in southern China, have done just that.

　　The Daya Bay experiment tracks electron antineutrinos produced by six nuclear reactors at Daya Bay. Two sets of detectors are used: one placed a few hundred metres from the reactors, and one placed 2 kilometres away. The more distant detectors see fewer electron antineutrinos than the nearby ones, because as they travel some change into other flavours of antineutrino, which the detectors cannot spot.

　　Larger than expected

　　The difference depends predominantly on theta13, says Luk, co-spokesperson for the experiment. Their result was announced on 8 March in a seminar at the Institute of High Energy Physics in Beijing. "Finally we know the size of theta13. It's not as small as first thought."

　　"The implication is huge," says Francis Halzen of the University of Wisconsin-Madison and the head of the IceCube Neutrino Telescope at the South Pole. The result means physicists can now build experiments to find out whether neutrinos behave differently from antineutrinos – which would have been difficult had theta13 been small.

　　Such experiments could provide clues as to why the universe had a preference for matter over antimatter in the moments after the big bang. Without such a bias, all the matter would have been annihilated by antimatter.

　　"Theta13 is as large as we could have hoped for – an amazing result that definitely puts neutrino physics on the fast track," says Halzen. "We now know that we can move to the next frontier."

　　On the hunt for shape-shifting neutrinos (Image: Roy Kaltschmidt, Lawrence Berkeley Nat'l Lab)

　　（原文链接：http://www.newscientist.com/article/dn21571-neutrinos-could-help-explain-missing-antimatter.html）