The DONUT team fired an intense beam of neutrinos, which they expected to contain tau neutrinos, at a target consisting of iron plates with layers of emulsion sandwiched between them. One in a million million tau neutrinos interacted with an iron nucleus to produce a tau lepton, which subsequently decayed leaving a characteristic track in the emulsion. The team isolated just four tracks containing the telltale kink of the tau neutrino track, out of a possible six million.

Far from completing the picture for neutrino physics, the new discovery poses many more questions, such as whether the tau neutrino has mass, and whether it interacts differently to the other neutrinos. The answers to these questions will have major implications for our understanding of the evolution of the universe, and international efforts to find them are already underway.