Direct Observation of the NU Tau
The name DONUT is an abbreviation for "Direct Observation of the NU Tau," where the last two words refer to the Greek symbol representing the tau neutrino: ?t
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"Neutrinos are very difficult to work with. They are different than any other particles. They are sort of pure. It is very hard to do neutrino experiments, but I think they may be the first ones to show unexpected interactions."
The DONUT experiment spent six months to record data on two sets of emulsion plates, each set containing four plates. Emulsion plates are a sort of 3D-film to take pictures of particles. Scientists used CCD cameras and computers to analyze the emulsion plates, which contain millions of tracks, eventually finding 4 events caused by tau neutrinos.
In 1953, the first team of scientists trying to demonstrate the reality of neutrinos, until then a completely hypothetical particle, called their experiment "Project Poltergeist," refering to the ghost-like properties of these particles. For his work in this and a follow-up experiment, Fred Reines received the 1995 Nobel Prize.
"We would watch patiently and catch one every few hours or so. And there were many hours available for watching in a month or a year."
Clyde Cowan, remembering the discovery of the electron neutrino in 1956
Because neutrinos interact with matter much less than light particles (photons), neutrinos escape cosmic explosions much quicker than light. In the case of supernovae 1987a, the first supernovae recorded by modern telescopes, neutrinos hit the earth about eight hours before the first light arrived. Two underground experiments in the U.S. and Japan recorded a total of about 20 neutrinos of the more than 1057 neutrinos that scientists estimated to have left the collapsing star.
The same way you can overexpose a film in your camera, you can overexpose emulsion plates by having too many charged particles crossing them. If the magnets used for sweeping the neutrino beam line free of charged particles would have failed once, only for a fraction of a second, all data taking by the DONUT experiment would have been erased due to overexposure.
"I have hit upon a desperate remedy to save ... the energy theorem." Wolfgang Pauli, introducing in 1930 the concept of a new, invisible particle to save the law of energy conservation for nuclear beta decays. Enrico Fermi later named the particle the `neutrino.'
"Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
...
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
..."
John Updike, in: From Telephones Poles and other Poems
Additional reading:
The Elusive Neutrino, by Nickolas Solomey (Scientific American Library, 1997, New York).
Standard Model of
Elementary Particles and Forces
Physics experiments of the past hundred years have revealed the atom's structure, with its nucleus and orbiting electrons. Today's experiments use powerful particle accelerators to explore the deepest substructure of matter, the particles and forces inside the proton and neutron of the atom's nucleus. Decades of research have now given us a remarkably simple theoretical model of the elementary particles and forces of matter. The discovery of the top quark at Fermilab in March, 1995, provided strong evidence for the Standard Model, the prevailing theory that describes the elementary particles and forces. These particles are the matter particles called leptons and quarks; and the force-carrying particles called bosons.
The six leptons include the electron, the muon, and the tau; and three neutral particles postulated to be massless --the electron neutrino, the muon neutrino, and the tau neutrino.
The six quarks include the up and down quarks that make up the proton and neutron, as well as the strange, charm, bottom, and top quarks that were present at the birth of the universe and that we now produce in particle collisions.
The gauge bosons include the gluon that transmits the strong force that holds quarks together in the nucleus; the W and Z bosons that transmit the weak nuclear force responsible for radioactive decay; and the photon that transmits electromagnetic force.
Physicists Find First Direct Evidence for Tau Neutrino at Fermilab^^
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