Fermilab Education Home sciencelines Fall 2000

Tau Neutrino Evidence Announced at Fermilab

This summer scientists at Fermi National Accelerator Laboratory announced the first direct evidence for the subatomic particle, the tau neutrino. The tau is an almost massless particle that carries no electric charge and barely interacts with surrounding matter. Previous experiments showed indirect evidence for its existence, but it had not been observed directly as yet. The tau is the third neutrino of the Standard Model of elementary particles, a theoretical description that groups all particles into three generations.

The first electron neutrino was discovered in 1956, the muon in 1962.

The Fermilab experiment responsible for the announcement is the Direct Observation of the Nu Tau (DONUT) experiment. DONUT is a collaboration of physicists from the U.S., Japan, Korea, and Greece. In 1997, using Fermilab's Tevatron accelerator, the scientists produced an intense neutrino beam. They expected this beam to contain the tau neutrinos they were hunting. The beam crossed through a three-foot-long detector, sort of a DONUT target of iron plates alternating with layers of emulsion in which the particle interactions could be recorded.

Through the experiment the detectors recorded a total of six million potential interactions. Yet after three years of intense work to identify the tracks, the scientists reported four instances of a neutrino interacting with an atomic nucleus to produce a charged particle called a tau lepton, the signature of a tau neutrino. "We finally have direct evidence that the tau neutrino is one of the building blocks of nature and that it reacts with other particles in accordance with our current scientific theory of particle interactions," said Byron Lundberg, Fermilab physicist and DONUT spokesman.

For first-hand information about the experiment go to: http://www.fnal.gov/pub/inquiring/physics/neutrino/discovery/donut_details.html

"Research of the past century has revealed the structure of the atom with its nucleus and orbiting electrons. Probing still deeper into this structure, research now focuses on the individual particles inside the nucleus, studying their properties and the ways they interact. A continuing series of experiments has resulted in a model of the fundamental particles and forces of matter, the Standard Model."

To read more on the Standard Model see: www.fnal.gov/pub/inquiring/matter/madeof/index.html