At last, evidence of the top quark.

There was no eureka moment, no sudden revelation. Just the slow, painstaking accumulation of data from the debris of trillions of highenergy collisions between protons and their antimatter counterparts. In the end, the evidence favoring the existence of the long-sought top quark proved strong enough to war rant an announcement.

This week, physicists at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., presented the first direct experimental evidence for the top quark. "Until now, there was not even a hint of top," says William C. Carithers of the Lawrence Berkeley (Calif.) Laboratory, speaking for the team of about 440 physicists using the Collider Detector at Fermilab (CDF). "Now, we're starting to see a hint that it's really there."

Quarks come in a variety of flavors. Two kinds of quarks - known as up and down quarks - combine to create the protons and neutrons of everyday matter. Three other types of quarks -- strang, charm, and botton -- have been found in particle accelerators, and at least one of these types may exist at the center of extremely dense, massive stars.

For a variety of reasons, theorists have long believed that a sixth quark, much more massive than the others, ought to exist to complete the quark family of subatomic particles. With the 1977 discovery at Fermilab of the bottom quark, the search was on for the top quark.

Now, researchers using Fermilab's powerful Tevatron accelerator may have glimpsed the top. Created as pairs of top quarks and antiquarks in collisions between protons and antiprotons moving at nearly the speed of light, these massive particles disintegrate almost immediately in ways that create a shower of other particles that can then be detected.

The team spent nearly a year sifting through 1 trillion collisions to find 7 million events showing interesting particle interactions. Of these, 20,000 involved particles known as W bosons -- one of the types of particles into which the top can decay. The final selection produced 12 events, representing three ways in which top quarks decay into other particles.

Because various subatomic particles can decay in ways that mimic those of the top quark, researchers have to be careful to distinguish these "background" decays from top quark decays. "We can't rule out the possibility that what we're seeing is a rare fluctuation in the background," Carithers says. But "we estimate that chance as about 1 in 400."

From their results, the researchers conclude that the top quark has a mass (expressed in energy units) of 174 billion electronvolts, with an uncertainly of about 10 percent. This makes the top quark nearly as heavy as a gold atom. The measured mass is also consistent with recent theoretical predictions.

The CDF team has submitte a 153-page paper describing its results to PHYSICAL REVIEW D. "The analysis is extremely complex," says Melvyn J. Shochet of the University of Chicago. "It was absolutely essential that we understand all the details . . . before we went public and presented it for publication."

"I'm persuaded that the evidence is very strong," says John Peoples Jr., Fermilab director. Still, researchers operating the DZero detector at Fermilab have failed so far to confirm the existence of the top quark, although they have seen a handful of "interesting" events. "We're not claiming a discovery," Carithers notes. "We're in that middle ground where the excess of events that we see is too large to ignore but too small to cry eureka."

At this point, "it's important to collect more data," Shochet says. Indeed, the current experiment has already yielded at least one additional event that may have involved a top quark.