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Lepton physics work attracts Nobel honors.

Along with the electron and muon, the tau particle and three varieties of neutrinos are now firmly established as members of the lepton group of subatomic particles. This year's Nobel Prize in Physics, announced last week, honors two physicists who played key roles in experiments demonstrating the existence of these constituents of matter.

Frederick Reines of the University of California, Irvine, contributed to the discovery of the neutrino in the 1950s, while Martin L. Perl of the Stanford (Calif.) Linear Accelerator Center (SLAC) was a member of the research team that identified the tau lepton in the 1970s.

The neutrino originated as a hypothetical particle, invented in 1930 by Wolfgang Pauli to account for some missing energy when a radioactive atomic nucleus emits an electron. To uphold the law of conservation of energy, he proposed the existence of an uncharged subatomic particle that accompanied the electron and interacted very little with other forms of matter.

Subsequent theoretical developments bolstered the credibility of Pauli's hypothesis, but most physicists despaired of detecting such an elusive particle.

Realizing that nuclear reactors could serve as intense neutrino sources, Reines, working with Clyde L. Cowan Jr., set up an experiment to find neutrinos by looking for the rare instances in which a certain kind of neutrino collides with a proton (in water) to create a neutron and positron. They eventually accumulated enough experimental data to prove the existence of the neutrino as a free particle. Cowan died in 1974.

That year, Perl was finding hints in the debris of collisions between high-energy electrons and positrons of a hitherto undiscovered lepton. Although some theorists had suggested that heavy leptons exist, no one was certain that any would be found (SN: 9/12/92, p.174).

After more than a year of data analysis, Perl persuaded his coworkers at the Stanford Positron-Electron Asymmetric Ring (SPEAR) particle collider that they were truly observing a new and different type of elementary particle. He dubbed it the tau.

About 3,500 times heavier than the electron, the tau lepton is a member of the same family of subatomic particles as the top and bottom quarks (SN: 7/1/95, p.10).
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Title Annotation:Frederick Reines and Martin L. Perl win 1995 Nobel Prize in Physics
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Oct 21, 1995
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