A DONUT to find the tau neutrino
26 July 2000
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On July 21, the DONUT team at the Fermi National Accelerator Laboratory in Illinois announced the first direct observation of the tau-neutrino. This elusive particle is the uncharged partner to the tau, the third and heaviest of the electrically charged members of the lepton family of particles which includes the familiar electron. The tau, which is approximately 3600 times heavier than the electron was discovered in 1975, and physicists have since assumed the existence of its partner, the tau-neutrino. However, until the announcement by the DONUT team no experiment had demonstrated that the neutral particles associated with taus could interact to produce taus - the sure sign of the unique relationship between the particles.
The DONUT (Direct Observation of the Nu Tau) detector includes a 1-m long target of iron plates sandwiched with layers of nuclear to record particle interactions. In the target, one out of one million million tau-neutrinos interacted with an iron nucleus to produce a tau lepton, which left its one-millimeter-long tell-tale track in the emulsion. This image shows one of spokesman for the experiment, Vittorio Paolone, University of Pittsburgh, inside the coil of the electromagnet used to eliminate charged particles from the neutrino beam.
Credit: Fermi National Accelerator Laboratory