Extra data bolster top quark discovery.

The top quark's true identity is beginning to emerge from the debris of trillions of high-energy collisions between protons and their antimatter  counterparts.

Last year, physicists at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., presented the first direct experimental evidence for the existence of the long-sought top quark (SN: 4/30/94, p.276). But significant uncertainties in the limited data available rendered the results suspect.

Now, after acquiring and sifting through three times as much data, Fermilab researchers are confident that they have indeed discovered the top quark. Moreover, teams using two different detectors have observed traces of this elusive subatomic particle. A year ago, only the group using the Collider Detector at Fermilab (CDF) claimed any sightings.

At that time, researchers at the rival DZero detector "just did not have enough events to make a statement about the top quark's existence, but now, with a larger data sample, the signal is clear," says Paul D. Grannis of the State University of New York at Stony Brook, a DZero team member.

Representatives of the CDF and DZero teams reported their findings at a seminar held last week at Fermilab.

According to the standard model of particle physics, quarks come in six varieties. 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 -- strange, charm, and bottom -- have been found using particle accelerators. The newly discovered top quark completes the family.

Created as quark-antiquark pairs in collisions between protons and antiprotons moving at nearly the speed of light, top quarks disintegrate almost immediately to create characteristic showers of other particles that physicists can then detect. However, because decays of other subatomic particles can mimic those postulated for the top quark, researchers must take care to distinguish these "background" events from genuine top quark decays.

Out of trillions of collisions, the CDF team identified about 40 events as top quark candidates, significantly more than the 12 events expected as background. From these data, the physicists calculated that the top quark has a mass (expressed in energy units) of 176 billion electronvolts (GeV), with an uncertainty of 13 GeV.

"The new mass estimate closely matched our previous measurement," says Paul Tipton of the University of Rochester and a member of the CDF group. "This was very reassuring and made us believe the result that much more."

The DZero group has found about 17 candidate events, leading to a mass estimate for the top quark of 199 GeV, with an uncertainty of 30 GeV. "There's a good overlap between the measurements," Tipton says.

"The detectors are vastly different, and what you record is very different, but the physics of producing the particles and what those particles do in the detectors is the same," adds DZero team member Andrew R. Baden of the University of Maryland in College Park. "I think it's pretty clear now that [both detectors] are looking at the same phenomenon."

The two teams plan to continue collecting data until the end of the year. "By then, we'll have twice the data we have now," Baden says.

Much effort will go into reducing the uncertainty in the top quark mass. As the number of candidate events increases, researchers will also begin to study other characteristics of the top quark.

"We have this new particle, and we're the only people in the world who have what is, in effect, a new laboratory," Tipton says. "There are possibilities for observing new physics in both how the top is produced and how it decays."