The Most Wanted Particle

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Imagine an infinitesimal particle that is as heavy as a large atom and less tangible than a shadow. For 15 years, hundreds of physicists have been chasing such an improbable phantom. Their quarry is the top quark, the sole missing member of a family of subatomic particles that form the basic building blocks of matter. Of six types of quarks that are believed to exist, five have already been discovered. "The top," says Harvard University theorist Sheldon Glashow, "is not just another quark. It's the last blessed one, and the sooner we find it, the better everyone will feel."

Physicists will celebrate because the top is the absent jewel in the crown of the so-called Standard Model, a powerful theoretical synthesis that has reduced a once bewildering zoo of particles to just a few fundamental constituents, including three whimsically named couplets of quarks. Up and down quarks combine to create everyday protons and neutrons, while charm and strange quarks make up more esoteric particles, the kind produced by accelerators and high-energy cosmic rays. In 1977 physicists discovered a fifth quark they dubbed bottom, and they have been looking for its partner, top, ever since. Not finding it would amaze and befuddle particle physicists. Without the top, a large chunk of the theoretical edifice, like an arch without a keystone, would come crashing down.

Scientists have long suspected that top quarks are routinely produced by the powerful collider at Fermi National Accelerator Laboratory near Chicago. So far, however, a thicket of more ordinary particles has concealed them from view. But the top may not elude discovery much longer. In late October, researchers at Fermilab's Collider Detector found a provocative set of tracks hinting that a top may have briefly materialized, then vanished like a Halloween ghost. The tantalizing event was reported at a conference held at the facility in mid-November. Since then, physicists have talked of little else.

Theorists have already deduced that the top quark is heavier than any known particle. "A single top quark," exclaims Fermilab physicist Alvin Tollestrup, "probably weighs at least as much as a whole silver atom does." (With an atomic weight of 108, a silver atom is made up of hundreds of up and down quarks.) Exactly how much top quarks weigh is a question scientists are anxious to answer, but first they must find some to measure -- a task considerably complicated by the fact that in nature these massive but ethereal entities made only a cameo appearance, just after the Big Bang.

Top quarks emerged from the primordial radiation "around a thousandth of a billionth of a second after the Big Bang," estimates University of Michigan theorist Gordon Kane. But as the early universe expanded and cooled, they vanished. Their fleeting existence left behind a fundamental puzzle that physicists are struggling to solve: What makes some particles so massive while others -- photons, for example -- have no mass at all? Because of its boggling heft, the top quark should help illuminate what mysterious mechanisms -- including perhaps other, still weightier particles -- are responsible for imparting mass, and hence solidity, to the physical world.