Tracking Down the Top Quark

March 07, 1995

The ancient Greeks were the first to speculate that all matter was made up of invisible, irreducible particles called atoms. They envisioned the atom as a tiny, hard sphere that constituted the smallest possible division of any substance.

Yet the existence of atoms wasn't put on a firm scientific basis until the discovery of the periodic table of elements in the 19th century and the realization the atom was composed of still smaller units such as protons, neutrons and electrons. These advances made possible the modern science of chemistry and the dawn of the nuclear age.

Now two teams of physicists at the Fermi National Accelerator Laboratory in Illinois have taken the Greeks' idea one step further with convincing evidence that the heavier subatomic particles -- protons and neutrons -- are constituted out of still smaller bits of matter called quarks.

The existence of quarks was first suggested in the 1960s by physicist Murray Gell-Mann, who theorized there should be six types or "flavors" of quarks if the widely accepted Standard Model of nuclear structure were correct. In the interveningyears researchers managed to identify five of the theorized particles in the laboratory, but it wasn't until last week that scientists finally pinned down evidence for the existence of the sixth particle, the so-called "top" quark. That discovery holds out the prospect of an eventual grand unification theory that would relate matter to the four known fundamental forces of the universe.

The accelerator at Fermilab is currently the most powerful in the world, but it cannot reach the energy levels required to determine whether even more exotic particles exist, such as the hypothesized "Higgs boson," which is thought to endow particles with the property of mass. That was to be the task of the Superconducting Supercollider, which Congress killed last year. As a result, leadership in particle physics is likely to shift to the European CERN lab near Geneva, Switzerland. Ironically, though the American discovery of the top quark was an undoubted triumph of technology and perseverance, it also may turn out to be the last hurrah for American leadership in a field the United States once dominated.

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