The OPERA experiment at Gran Sasso has observed a third neutrino oscillation, with a muon-neutrino produced at CERN detected as a τ neutrino in the Gran Sasso laboratory. This extremely rare event was observed only twice previously.
OPERA, which is run by an international experiment involving 140 physicists from 28 research institutes in 11 countries, was set up for the specific purpose of discovering neutrino oscillations of this kind. A beam of neutrinos produced at CERN travels towards the INFN Gran Sasso National Laboratory some 730 km away. Thanks to their weak interactions, the neutrinos arrive almost unperturbed at the giant OPERA detector, which consists of more than 4000 tonnes of material, has a volume of some 2000 m3 and contains nine million photographic plates. After the first neutrinos arrived at Gran Sasso in 2006, the experiment gathered data for five consecutive years, from 2008 to 2012. The first τ neutrino was observed in 2010, the second in 2012.
The arrival of the τ neutrino is an important confirmation of the two previous observations. Statistically, the observation of three τ neutrinos enables the collaboration to claim confidently that muon neutrinos oscillate to τ neutrinos. Data analysis is set to continue for another two years.
