Gotcha!

By J. Madeleine Nash/Chicago

It was once called the great white whale of physics. The longer the "top" quark managed to elude capture, the more obsessed its pursuers became with the importance of hunting it down. For in the subatomic world, the top was, scientists believed, the sixth and last of the quarks -- pointlike particles that constitute the basic building blocks of matter. As the years passed, failure to find the top became a source of consternation and potential embarrassment to the theorists who swore it must exist.

But last week the scientific equivalent of "thar she blows" echoed around the world. The news came from several hundred particle hunters working at Fermi National Accelerator Laboratory near Chicago, who presented compelling evidence that not one but 12 top quarks had briefly surfaced inside a mammoth detector in their lab.

The first sightings of this long sought trophy have still to be confirmed, but when they are, they will culminate one of the richest periods of discovery in the history of science. They will also justify the confidence physicists have placed in the so-called Standard Model, a powerful theoretical edifice that has reduced a once bewildering array of subatomic particles to just a few fundamental constituents. These include three pairs of light particles known as leptons, of which the negatively charged electron and chargeless neutrino are the most familiar, and three pairs of heavier particles known by the whimsical name of quarks. "Up" and "down" quarks combine to create protons and neutrons, the components of everyday matter, while "charm" and "strange" quarks conspire to make more exotic particles, the sort produced in deep space by quasars and high-energy cosmic rays. In 1977, when a fifth quark called "bottom" was discovered, physicists quickly deduced that it too must have a partner.

That partner turns out to be well worth years of searching. Its apparent characteristics contain intriguing hints of an unexplored microcosmos, one that may be populated by particles far odder than any discovered to date. For the top quark is extraordinarily heavy. It is, to be exact, 200 times heavier than a proton and almost as hefty as an entire atom of gold. That an elementary particle can weigh so much, says University of Chicago physicist Henry Frisch, amounts to a "tantalizing clue." It suggests that the top is intricately entwined with the mysterious mechanism that is responsible for creating mass.

Why are some particles, like the top quark, so heavy, while others, like the photon, have no mass at all? The favored explanation for this striking asymmetry invokes a still hypothetical class of particles known as Higgs bosons, which are imagined to suffuse the universe like a dense fog. The force exerted by this field of Higgs particles can be loosely compared to the tug of gravity. A massless photon navigates through the Higgs field as though it were not there, while other particles experience such great drag that they, in essence, gain weight. Knowing the mass of the top quark should help flesh out the very preliminary sketch theorists have made of the Higgs boson and suggest to experimentalists clever ways of looking for it. There is even a chance, some physicists speculate, that the Higgs will turn out to be an odd couplet made up of the top and its antimatter twin. "Is the top the Yeti?" wonders Fermilab theorist Christopher Hill, referring to the mythical creature that is said to inhabit the high Himalayas, "or is it the footprint of the Yeti? We - don't know the answer to that question. What we do know is that the Standard Model is incomplete."

Nearly two decades ago, when physicists started designing the collider detector at Fermilab (CDF), they had no idea the search for the top would drag on for so long. Theorists predicted that the top should be no more than three times the size of its partner, bottom, putting it well within the range of particle accelerators then available in both the U.S. and Europe. In 1984 Carlo Rubbia and his collaborators at CERN, the European center for nuclear research near Geneva, Switzerland, claimed to have discovered the top, but that turned out to be a mistake. By the end of 1990, other accelerators had all but dropped out of the top hunt save for Fermilab's Tevatron, then as now the most powerful collider.

To trap the top quark required the sustained effort of 440 physicists from 36 institutions in five countries. They spent six years building a gigantic detector, a mass of steel and electronics that weighs five tons and stands more than three stories tall. This ungainly contraption sits inside the four- mile circular tunnel of the Tevatron; and in the detector's hollow center, protons and antiprotons, accelerated to nearly the speed of light, smash into one another many thousands of times a second. The enormous energies unleashed by these collisions create sparkling showers of short-lived particles whose tracks flicker across computer screens. Searching among these streaks, scientists finally spotted, they believe, the top quark, one of nature's earliest and most ephemeral creations.

The original top quarks supposedly emerged from the roiling sea of primordial radiation less than a trillionth of a second after the Big Bang. Then, as the universe expanded and cooled, they all but disappeared. Now they occur naturally only under certain conditions. To conjure them up, scientists have to re-create the fiery conditions that followed the Big Bang, not an easy task. Because the top is so heavy, only the most energetic collisions in the Tevatron are capable of producing the particle at all. In addition, this king of quarks has such an infinitesimal lifetime that its presence can be inferred only from the whispery contrails of other particles into which it promptly decays. Thus the detector designed by Fermilab's scientists consists of more than 100,000 components, each intended to track different types of particles. A superconducting magnet, for example, helps measure the energy of electrons % and muons. The less these charged particles are bent by the electromagnetic field, the more energetic they are -- and the more likely that they were created by a top quark.

Among the CDF's most vital parts are the fast electronics that sift through torrents of incoming data, instantaneously separating the mundane from the rare. "We're looking for needles in haystacks," observes University of Michigan physicist Myron Campbell, "and to find them, we have to process a haystack every second." During the last experimental run, for instance, a trillion collisions between protons and antiprotons occurred inside CDF's big particle trap. Yet of these, only 16 million were deemed promising enough by the detector's electronic gate-keepers to be worth more detailed analysis. Further winnowing occurred as banks of computers examined myriad measurements associated with each collision, flagging only the most interesting. Out of all this, a dozen candidates for the top emerged. "If collisions were dollars, it's like starting with the entire federal budget," quipped William Carithers Jr., a physicist at the Lawrence Berkeley Laboratory in California, "and ending up with 12 bucks."

The search for the top quark taught its hunters the true meaning of the word marathon. "This has been a part of my life for so long," says Harvard University physicist John Huth, "that there's a sense of exhaustion." The time scientists once spent working with the detector is now consumed by meetings, some 20 a week.When the CDF team comes together, it is so large it must convene in the Fermilab auditorium, and the result sometimes resembles pandemonium. The 152-page paper reporting evidence for the top quark was sent off to the Physical Review two weeks ago. It could have been submitted two months ago, but questions erupted from members of the collaboration that triggered further soul searching and the insertion of more caveats.

Even now, some of CDF's scientists fret that they have overlooked some fatal flaw. They believe there is still 1 chance in 400 that they could be wrong, which seems extremely small to laypeople. But it is sobering to remember that odds that seem like a sure bet at a racetrack are not enough to support scientific claims. Over the coming months, the lingering uncertainty that surrounds last week's announcement should be dispelled as more data are collected, not just by CDF but by a rival detector that goes by the name of DZero. If the top really is out there, as most physicists believe, then it will gradually come into focus. If not, even greater excitement will ensue. For if the top quark is not creating those bursts of particles deep inside the detector, then what is?