Physicists have identified the production of the elusive single top quark, two research teams report.

Previously top quarks have been observed only when produced in pairs, as when they were initially discovered 14 years ago at the Fermi National Accelerator Laboratory in Batavia, Ill. Now, researchers using Fermilab’s two detectors announced March 9 that they have detected single top quarks. The techniques used to find the singleton quarks could help to identify other rare particles, such as the Higgs boson, the scientists say.

“What a discovery,” comments Nobel laureate David Gross of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. “These researchers should be congratulated.”

Quarks are fundamental particles of matter that come in six varieties known as “flavors.” Ordinary matter consists mostly of two quark flavors, the “up” and “down” quarks that make up protons and neutrons. Other quarks are found in exotic subatomic particles or are created in high-energy collisions in particle accelerators. The top quark was the last flavor to be discovered experimentally, in 1995.

Since that time, the two groups at Fermilab, using the CDF and the DZero detectors, have combed through data from billions upon billions of particle collisions, looking for the unique features that would signal a single top quark.

But only about one in every 20 billion collisions produces a single top quark, and that weak signal easily gets lost in the background of other particle debris. “Pairs of top quarks have a more distinctive signature than singles, and so are easier to distinguish,” explains physicist Darien Wood, a spokesman for the DZero group.

The two groups had slightly different methods of analyzing the data, which allowed for healthy competition, says physicist Robert Roser, a spokesman for the CDF group.  

A possible sighting of a top quark singleton in 2007 indicated that the scientists were on the right track. “But the evidence has to be solid to claim a discovery,” says Jacobo Konigsberg, a spokesman for CDF. “More data was needed to increase the statistical significance of our data.” 

The sophisticated data analysis used to find the singleton top quarks could help scientists make sense of the results of other particle collision experiments, the researchers say. “The single top is the first stepping stone to other unknowns, like the Higgs boson,” says Roser.

The Higgs is a so-far hypothetical particle that most theorists believe is necessary to endow other subatomic particles with mass. Its discovery is being pursued at Fermilab and is also a prime goal for the Large Hadron Collider, a particle accelerator near Geneva scheduled to begin particle collisions later this year.

“Succeeding in isolating single top quarks is a sign that the community is up to the Herculean task of finding the Higgs boson,” Gross says.

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