Scientists from the OPERA experiment at INFN's Gran Sasso National Laboratory have announced the first direct observation of a neutrino transforming from one type into another. When confirmed by a few more such events, this observation will provide further evidence that neutrinos have mass, a phenomenon that remains unexplained by physicists' recipe for understanding the universe, the Standard Model.
Neutrinos are very light, neutral particles that exist in three types: electron, muon and tau. Over the past 15 years, several experiments have shown that neutrinos can spontaneously change type, or oscillate, as they travel long distances. These experiments, however, all measured the disappearance of certain types of neutrinos, leaving open the question of whether oscillation is a one-way or a two-way process. OPERA is the first experiment to measure the appearance of a tau neutrino in a beam of muon neutrinos, thus confirming that neutrinos oscillate between different types.
The scientists of the OPERA experiment have spent the last three years searching for the appearance of a tau particle in their detector as a beam of muon neutrinos, created 732 kilometers away at CERN in Geneva, Switzerland, passed through their underground experiment. The appearance of a tau particle in their detector indicates that one of the billions upon billions of muon neutrinos transformed into a tau neutrino.
More details can be found in the INFN press release and CERN press release, or you can watch this video from CERN that describes neutrinos and the oscillation phenomenon, the OPERA detector and creation of the muon neutrino beam, and today's announcement.