Editor's note: This article was originally published in the September 1997 issue of Scientific American (a PDF version of the original is available for purchase below). We have resurfaced this article to commemorate the end of the Tevatron.

In March 1995 scientists gathered at a hastily called meeting at Fermilab—the Fermi National Accelerator Laboratory in Batavia, Ill., near Chicago—to witness a historic event. In back-to-back seminars, physicists from rival experiments within the lab announced the discovery of a new particle, the top quark. A decades-long search for one of the last missing pieces in the Standard Model of particle physics had come to an end.

The top quark is the sixth, and quite possibly the last, quark. Along with leptons—the electron and its relativesquarks are the building blocks of matter. The lightest quarks, designated “up” and “down,” make up the familiar protons and neutrons. Along with the electrons, these make up the entire periodic table. Heavier quarks (such as the charm, strange, top and bottom quarks) and leptons, though abundant in the early moments after the big bang, are now commonly produced only in accelerators. The Standard Model describes the interactions among these building blocks. It requires that leptons and quarks each come in pairs, often called generations.

Physicists had known that the top must exist since 1977, when its partner, the bottom, was discovered. But the top proved exasperatingly hard to find. Although a fundamental particle with no discernible structure, the top quark turns out to have a mass of 175 billion electron volts (GeV)—as much as an atom of gold and far greater than most theorists had anticipated. The proton, made of two ups and one down, has a mass of just under 1 GeV. (The electron volt is a unit of energy, related to mass via E = mc².)

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