OPERA – the Oscillation Project with Emulsion-tRacking Apparatus experiment at the INFN Gran Sasso Laboratory – has detected the fifth occurrence of a tau neutrino (ντ). Setting out from CERN as a muon neutrino (νμ), the particle was detected at Gran Sasso as a ντ after travelling 730 km through the Earth. This detection of a fifth ντ firmly establishes the direct observation of the transition from νμ to ντ, with a statistical precision of 5σ, the now standard threshold for a discovery in particle physics.
The international OPERA experiment, which involves about 140 physicists from 26 research institutes in 11 countries, was designed to observe this exceptionally rare phenomenon, gathering data in the neutrino beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project (CERN Courier November 2006 p24). A small fraction of the incoming neutrinos interacted with the giant detector, consisting of more than 4000 tonnes of material, with a volume of some 2000 m3 and some nine million photographic plates, to produce the particles observed. After detecting the first few νμ produced at CERN in 2006, the experiment has collected data for five years, from 2008 to the end of 2012. The first ντ was observed in 2010. The second and third events were reported in 2012 and 2013, respectively, while the fourth one was announced in 2014 (CERN Courier May 2014 p9).
The OPERA collaboration will continue to analyse the data collected, searching for other νμ to ντ transitions, and possibly also measure the oscillation parameters, for the first time using oscillated ντ.