OBSERVATION OF THE TAU NEUTRINO

The τ neutrino was the last standard-model fermion to be directly observed. We report on the strategy, design, and results of the DONuT experiment. The data were acquired at Fermilab during 1997, and in July 2000, the DONuT collaboration presented four τ-neutrino interactions. Modern nuclear-emulsion processing was the key technology that made it possible to detect the decay of τ leptons produced in charged-current interactions of τ neutrinos. The precision of the emulsion data and the speed of digitization enabled the use of novel methods to locate interaction vertices and search for decays in emulsion data. We also discuss the level of background in the τ signal, which is small enough to permit the identification of individual τ events with high confidence.