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Z particle: last link that joins 2 forces of matter

By Robert C. CowenNatural science editor of The Christian Science Monitor / May 13, 1983



Physicists who study the fundamental structure of matter are preparing to celebrate a ''golden jubilee'' year of discovery. This is confirmed by a preliminary report of the eagerly expected Z particle. It is the companion of the W particle found earlier this year in experiments at the European Center for Nuclear Research (CERN) at Geneva. Appearance of the Z particle, also in a CERN experiment, further demonstrates the underlying unity of two basic natural forces: the electromagnetic force between electrically charged particles and the ''weak'' force involved in radioactive decay.

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Thus, some 50 years after Enrico Fermi laid the foundation that prepared the way for this unifying theory, physicists now believe they have made a major advance in understanding the basic material forces. Frank Close of Britain's Rutherford Appleton Laboratory, writing in anticipation of these discoveries, said last January in Nature that ''1983 seems destined to be a golden jubilee. . . .''

As physicists now understand them, there are four basic material forces. Besides the electromagnetic and weak forces, there are the ''strong'' force, which holds atomic nuclei together, and gravity, which holds the universe together. Although physicists cannot yet prove it, many now suspect that these seemingly distinct forces are aspects of one great underlying principle.

The theory of ''electroweak'' unity, as it is called, was developed mainly by Sheldon Lee Glashow at Harvard University, Steven Weinberg of the University of Texas, and Abdus Salam at Imperial College, London.

Strong, but indirect, evidence had emerged during the past decade to support this theory. The theory predicted the existence of the W and Z particles. Thus it was crucial to prove that such particles really do exist if the theory were to be confirmed.

These particles mediate actions of the weak and electromagnetic forces. There should be two kinds of W's, one carrying a positive electric charge and the other a negative charge. The Z particle should be electrically neutral - hence its full name Z-zero.

The first W's were provisionally reported by CERN in January. Since then, a dozen of them have been found. What is more, they have the electric charge and the mass that scientists expected. Now an international research team has evidence of the Z, which also has the mass expected.

The latest discovery was provisionally reported by Carlo Rubbia of Harvard University, a member of the CERN team, during a recent trip back to the US. If confirmed, this finding, together with the W particles, will be considered one of the outstanding discoveries of this century.

Physicist Abraham Pais of Rockefeller University, who heard Dr. Rubbia's presentation, calls the experimental result ''simply beautiful.'' He explains, ''What made the earlier discovery of the W such a moving moment was the confluence of theory and experiment.'' Now that confluence seems to be happening again. If confirmed, this discovery will lend support to the electroweak theory, he says.