It is the chameleon of the subatomic world, but after many years of searching, direct evidence of a neutrino changing from one type into another has been found.
Neutrinos are thought to "oscillate" between three types: electron, tau and muon. But nobody has seen one after it has transformed.
For three years, a source at CERN in Switzerland has been firing billions of muon neutrinos towards the OPERA experiment beneath the Gran Sasso mountain in Italy, 730 kilometres away.
Detecting the appearance of a tau neutrino amidst the barrage of muon neutrinos is extremely difficult. "You have to be sensitive to even one single neutrino that has been transformed," says OPERA spokesman Antonio Ereditato at the University of Bern in Switzerland.
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