This issue... Wow! Fermilab Confirms the Tau |
Wow! Fermilab Confirms the Tau!by Sallie J. Ortiz
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.
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 noticean 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 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.
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
"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 |
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www.pnl.gov/energyscience/ |