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Wow! Fermilab Confirms the Tau!

by Sallie J. Ortiz

Standard ModelFermi National Laboratory (Fermilab) announced on Friday, July 21, 2000, the confirmation of the first "direct" evidence for the subatomic particle called the tau (rhymes with wow) neutrino. Earlier experiments provided convincing "indirect" evidence for the particle's existence, but no one has previously directly observed the tau neutrino.

The tau is the third and last neutrino to be confirmed on the Standard Model of elementary particles, a theoretical description of the basic constituents of matter and the fundamental forces of nature. The first two neutrinos to be discovered were the electron and the muon.

Cosmic Gall

Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.

The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.

They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall.

And, scorning barriers of class,
Infiltrate you and me! Like tall
And painless guillotines, they fall
Down through our heads into the grass.

At night, they enter at Nepal
And pierce the lover and his lass
From underneath the bed—you call
It wonderful; I call it crass.

John Updike
Telephone Poles and Other Poems,
1963

The tau neutrino is a massless (or nearly massless) particle that carries no electric charge and barely interacts with surrounding matter. Millions of them pass through your body every day without notice—an observation that inspired John Updike to pen this playful poem about neutrinos in 1963.

The new direct evidence for the last neutrino of the Model does not begin to close the chapter on neutrino physics. Scientists are still eager to learn whether neutrinos have mass, a result that would put a crack in the Standard Model, leading to major changes in our picture of the evolution of the universe. Already, physicists from the Super-Kamiokande experiment in Japan presented data in 1998 that may indicate that muon neutrinos oscillate and have mass.

Particles with a Past

The electron neutrino was discovered in 1956 by Frederick Reines and Clyde Cowan of Los Alamos National Laboratory using the Hanford and Savannah River Reactors. They described this fleeting, baffling subatomic particle as "the most tiny quantity of reality ever imagined by a human being." Their experiments raised the neutrino from its status as a figure of the imagination to an existence as a free particle.

Frederick Reines and Clyde Cowan at Hanford

Reines and Cowan began their definitive and ground-breaking experimental detection at a reactor in Hanford, Washington, and later moved to the new Savannah River Plant reactor in South Carolina. Cowan is far left and Reines is far right, shown with the Hanford team in 1953. They called their experiment "Project Poltergeist."

The muon neutrino was discovered in 1962 by Leon Lederman, Melvin Schwartz, and Jack Steinberger at Brookhaven National Laboratory. The Nobel committee awarded them the 1988 Nobel Prize for Physics for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino. The experiment was planned when the three researchers were associated with Columbia University in New York, and carried out using the Alternating Gradient Synchrotron (AGS) at Brookhaven National Accelerator Laboratory on Long Island.

The Muon Neutrino Team

A group of scientists from Columbia University and Brookhaven National Laboratory performed the first accelerator neutrino experiment and demonstrate the existence of two species of neutrinos, the electron and the muon. Shown from left to right are J. Steinberger, K. Goulianos, J. Gaillard, N. Mistr, G. Danby, W. Hayes, L. Lederman, and M. Schwartz

In 1975, the tau lepton was discovered by a group of scientists led by Martin Perl at Stanford Linear Accelerator Center (SLAC), which was strong evidence that a third species, a tau neutrino, must also exist. The Nobel committee decided to share the 1995 Nobel Prize for Physics with both Perl and Reines for their discoveries, 19 years apart, of the tau lepton and the first neutrino.

Tracking the Elusive Tau

Twenty-five years after discovery of the tau lepton, a collaboration of 54 physicists from the U.S., Japan, Korea, and Greece finally confirmed the direct evidence of the elusive tau neutrino working at Fermilab on the Direct Observation of the Nu Tau (DONUT) experiment.

"It is one thing to think that there are tau neutrinos out there," said Byron Lundberg, spokesman of the DONUT experiment, "but to really look for the rare incidence of a tau neutrino hitting a nucleus and transforming into a tau lepton is a hard experiment to do."

First-generation electron neutrinos and their second-generation cousins, muon neutrinos, are easier to produce and detect than tau neutrinos. Experimenters identified them in 1956 and 1962 by recording neutrino interactions creating either electrons or muons. More than 30 years of technological advancement have now allowed physicists to observe the third-generation tau neutrino producing a tau lepton.

In 1997, scientists used the Fermilab Tevatron accelerator to produce an intense neutrino beam, which they expected to contain tau neutrinos. The neutrino beam passed through the three-foot-long DONUT target of iron plates sandwiched with layers of emulsion, which recorded the particle interactions. In the target, one out of one million million tau neutrinos interacted with an iron nucleus and transformed into a tau lepton, which left its one-millimeter-long tell-tale track in the emulsion. The tau lepton leaves tracks in the emulsion just as light leaves marks on photographic film, but in three dimensions. A track with a kink indicates the decay of the tau lepton shortly after its creation.

The DONUT experiment recorded a total of six million potential interactions. Like looking for a needle in a haystack, the physicists analyzed signals from various components of the 50-foot-long detector, winnowing out all but 1000 candidate events. Of these, four of the events provided evidence for the tau neutrino.

It took physicists three years of painstaking work to identify the tracks revealing a tau lepton and its decay, the key to exposing the tau neutrino's existence, before the announcement of direct evidence could be made.

This latest discovery means that now all three of these neutrinos have been discovered at DOE facilities. This research was supported by the Office of Science's Office of High Energy Physics.

Contact: Byron Lundberg, Direct Observation of the Nu Tau (DONUT) Experiment, lundberg@fnal.gov, (630) 840-2408

Related Information:

The Story of the Neutrino, Fermi National Laboratory

The DONUT Experiment, Fermi National Laboratory

Idee Fixe," Fermi News, June 30, 2000; The Next Target: The Fourth Neutrino and Maybe More," Mike Perricone, FermiNews, June 30, 2000.

The Standard Model of Elementary Particles and Forces, DONUT Experiment, Fermi National Laboratory

"Got Mass? Super-K Says Neutrinos Do," Energy Research News, May/June 1998.

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