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10/18/95

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The meaning of tau

Consider the marvelous diversity of the physical world. From rock-hard granite to a supple maple leaf, from the gossamer of a spider's web to the glistening sheen of a soap bubble and the wetness of rain, we are surrounded by literally millions of different kinds of materials, each with distinct mechanical, chemical, optical and electrical properties.

This exuberant variety can be explained by the interactions of just three fundamental particles. But they are not the familiar electron, proton and neutron, as physicists thought in the 1930s and '40s.

At that time, physicists were convinced they understood the basic building blocks of matter. But they had a puzzle to solve - they did not understand the forces that hold together the nucleus of the atom or the processes that caused radioactive decay. Finding a solution to this puzzle led to the field known as particle physics, as experimenters built massive machines to smash particles together violently in order to study the forces between them.

The answer to the puzzle eventually led to a new picture of the proton and neutron. Instead of constituting relatively simple objects, as scientists thought, it turned out that these particles are made up of two types of even smaller particles called quarks, which were given the names "up" and "down." As a result of this new picture, physicists realized that normal matter is made out of electrons and up and down quarks. (In addition, each particle has an anti-particle that is identical to it, except that it carries the opposite electric charge.)

The full story was completed with the discovery of one more particle, a nearly massless particle called the neutrino, first detected by Clyde Cowan and Frederick Reines (who shares this year's Nobel Prize in physics for this work). Many radioactive decays of heavy nuclei are due to processes in which a neutron disappears, producing a proton, an electron and a neutrino.

It took a tortuous path of discovery with a number of unexpected twists to reach this new picture of matter. Perhaps the most unexpected twist was the discovery that, in addition to the four particles seen in ordinary matter and its decays, there are two more families of particles that are exact copies of the previous set, except that all but the neutrino members are heavier and unstable. This realization brought the number of fundamental particles to a grand total of 12.

"We find that everything of a complicated nature is made from three basic families of particles," said Martin Perl, a physicist at the Stanford Linear Accelerator Center who received this year's Nobel Prize in physics for his discovery of the tau, one of the 12 fundamental particles. "Eventually, this will lead to a better understanding of matter, energy and time."

Perl's 1970 discovery of the tau was the first clue that a third family existed. But the first hint of the unexpected, and still unexplained, family structure came in the early 1930s.

The Japanese physicist Hideki Yukawa had suggested during that decade that a new type of particle, which he called a meson, was responsible for nuclear forces, and he even had made a rough calculation of its mass. Physicists searching for this particle among cosmic-ray-produced particles instead found evidence for a different particle, which is now called the muon.

The muon turned out to be just like an electron, except that it is more massive and hence unstable. At the time, the physicist I.I. Rabi - Martin Perl's thesis adviser - expressed his own and the physics community's puzzlement at the discovery of the muon by asking "Who ordered that?"

Scientists also realized there is a distinct type of neutrino associated with muons and their decays. Leon Lederman, Melvin Schwartz and Jack Steinberger received the Nobel Prize in 1988 for their 1962 experiment that convincingly demonstrated that the neutrinos associated with muons are different from those associated with electrons.

The pi-meson predicted by Yukawa in the 1930s was found in 1947, but it was not the only meson to be discovered. In fact, an embarrassing number of mesons and other particles that were massive and unstable cousins to the proton and the neutron were produced by the world's various particle accelerators. By the early 1960s close to 100 such particles had been found.

In 1964 theoretical physicists Murray Gell-Mann and George Zweig brought order to this sub-atomic chaos. They proposed that most of these exotic particles could be explained as different combinations of three even more basic particles, which Gell-Mann called quarks after a nonsense word in James Joyce's novel Finnegans Wake.

Two of these three quarks are constituents of ordinary matter. The third quark was called strange because it was a constituent of a group of particles with peculiarly long half-lives compared to others of similar masses.

In the late '60s Richard Taylor, a physicist at the Stanford Linear Accelerator Center (SLAC), together with Henry Kendall and Jerome Friedman of the Massachusetts Institute of Technology reported the first experimental evidence that protons were, in fact, made up of quarks. All three scientists received the Nobel Prize in 1990 for this achievement.

In 1974 another SLAC team, headed by center director Burton Richter, working at the Stanford Positron-Electron Asymmetric Ring (SPEAR) that slammed together electrons and their anti-particles, positrons, reported the discovery of particles that contain a fourth quark type, called charm, simultaneously with a group at the Brookhaven National laboratory, headed by Samuel C.C. Ting of MIT.

This fourth quark completed the second family of basic particles, and the quark model appeared to be complete. But this neat arrangement didn't last long. In the early 1970s Perl, also analyzing results from SPEAR began to see evidence that a new kind of particle was being created.

At first, only Gary Feldman, who had an office next to Perl, agreed with him. It took a year of laborious collection and analysis of data before Perl was able to convince the other collaboration leaders - Richter, and George Trilling and Gerson Goldhaber from the Lawrence Berkeley Laboratory - that they were in fact observing a new and different type of lepton. He dubbed the new particle "tau" - the first letter in the Greek word for third - because its existence indicated that there was a third family of fundamental particles.

The discovery was announced in 1975. The physics community responded to the announcement much like it had responded to the earlier discovery of the muon. Why? Because it ruined a recently established organization chart.

Perl has described the three following years as the worst time in his life because scientists at other accelerators around the world continued to report that they could not find any evidence of the tau. In large part they failed to find evidence because they were using different types of machines that could not produce taus as easily as SPEAR. Finally, technical improvements made it possible for scientists working at a West German accelerator near Hamburg, the Deutsches Elektronen Synchrotron, to confirm the tau's existence. Today, accelerators routinely make large numbers of these particles.

While Perl and his colleagues were waiting on tenterhooks for verification of the tau, a research group at the Fermi National Accelerator Laboratory in Batavia, Ill., led by Leon Lederman, found evidence for the existence of a fifth quark, the bottom quark, supporting the existence of a third family of particles. It was not until last year that the sixth quark, called the top quark, was finally discovered by scientists at Fermilab.

Discovery of the top quark has allowed scientists to organize all the known fundamental particles neatly, this time into three families. This organization is the basis for what scientists call the standard model.

Perl, for one, would like to discover additional families of particles.

"Why should there be just three families?" he asks. "There is no reason that we know. When I discovered the tau, I envisioned a fourth, a fifth and a sixth . . . an infinite series of families."

One of Perl's current research projects with scientists from Cornell University involves studying large numbers of tau particles to see if any of them are behaving in unpredicted ways that might give some clue to the existence of new, even more massive leptons. "So far we haven't seen anything," he reports.

There is some tantalizing evidence that three-particle families may be all there are. Experiments suggests that there are only three types of light neutrinos. If this is correct, then any additional particle families must be quite different from the current three, because each of these includes one of these neutrinos.

Investigation into the fundamental structure of matter is the most basic type of research. Although it has no immediately practical application, historically it has had important long-term impacts on society. Perhaps the most dramatic example was the discovery of nuclear energy. But such applications typically take decades to materialize.

When SLAC's Burt Richter is asked about the direct value of this research, he often replies with an anecdote from the 1800s: Michael Faraday, who performed pioneering studies of electricity, was visited by William Gladstone, while he was a member of the British Exchequer, or treasury department. When Gladstone asked the scientist the value of his studies, Faraday replied, "Sir, I do not know, but some day you will tax it."

-dfs-

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