The Most Wanted Particle

By J. MADELEINE NASH CHICAGO

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.

Finding the top is the sort of discovery of which Nobel dreams are made, and the pressure to be first has become particularly intense now that the Collider Detector has a competitor on its tail, a rival Fermilab detector that began generating its own data last May. The sense of urgency has intensified arguments among the Collider Detector's 400 experimentalists over how to interpret the whispery tracks that appeared in October inside the device, a conglomeration of electronics and steel that stands 3 1/2 stories tall and weighs 4,500 tons. Through its hollow center, protons and antiprotons, accelerated to nearly the speed of light, smash into one another thousands of times in a second. The energy unleashed creates showers of short-lived particles that scintillate like tiny sparklers. From the evanescent flashes recorded by the detector, physicists can reconstruct the identities of the particles that produced them.

In this case, scientists observed the transitory trails of four particles into which a top and its antimatter twin should occasionally decay. Or did they? One clue was the detection of a muon, a close relative of the electron. At least, it appeared to be a muon. The reason scientists aren't sure is that the portion of the detector responsible for tracking muons is segmented like an orange. "And with the malice often displayed by inanimate objects," says University of Chicago physicist Henry Frisch with a sigh, "this muon went right up a crack between the segments."

By far the largest problem confronting the scientists is the sort of statistical error that bedevils political pollsters. For while physicists sometimes base discoveries on a single observation, more often they must rely on multiple events to build a case. Thus a second intriguing event, recorded by the Collider Detector in December, has come under especially intense scrutiny. The discovery of the top will no doubt emerge gradually, believes Peter Galison, a science historian at Harvard. "The expanding circle of belief," he says, "must start inside the experimental collaboration and then widen to include the whole physics community." How long this process will take, no one can gauge, for it depends not only on scientific ingenuity but also on the whims of Lady Luck.