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.
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