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Fermilab collider experiments discover rare single top quark

Scientists at the Department of Energy's Fermilab have observed a new subatomic process. In particle collisions produced by the Tevatron collider, currently the world's most powerful operating particle accelerator, two teams of scientists found single top quarks. The discovery has significance for the ongoing search for the Higgs particle.

Previously, top quarks had only been observed when produced by the strong nuclear force. That interaction leads to the production of pairs of top quarks. The production of single top quarks, which involves the weak nuclear force and happens almost as often as the strong force production, is harder to identify experimentally. Now, scientists working on the CDF and DZero collider experiments at Fermilab achieved this feat, almost 14 years to the day of the top quark discovery at Fermilab in 1995.

A single top quark candidate event recorded by the DZero experiment. The single top decayed into a bottom quark, a muon and a neutrino.

A single top quark candidate event recorded by the DZero collaboration. The top quark decayed into a bottom quark, a muon, and a neutrino.

Searching for single-top production makes finding a needle in a haystack look easy. Only one in every 20 billion proton-antiproton collisions produces a single top quark. Even worse, the signal of these rare occurrences is easily mimicked by other "background" processes that occur at much higher rates.

"Observation of the single top quark production is an important milestone for the Tevatron program," says Dennis Kovar, associate director of the DOE Office of Science for High Energy Physics. "The highly sensitive and successful analysis is an important step in the search for the Higgs."

Discovering the single top quark production presents challenges similar to the Higgs boson search in the need to extract an extremely small signal from a very large background. Advanced analysis techniques pioneered for the single top discovery are now in use for the Higgs boson search at Fermilab. In addition, the single top and the Higgs signals have backgrounds in common, and collisions that produce single top quarks can mimic collisions events that create a Higgs particle.

To make the single-top discovery, physicists of the CDF and DZero collaborations spent years combing independently through the results of proton-antiproton collisions recorded by their experiments, respectively. Each team identified several thousand collision events that looked the way experimenters expect single top events to appear. Sophisticated statistical analysis and detailed background modeling showed that a few hundred collision events produced the real thing. On March 4, the two teams submitted their independent results to Physical Review Letters (CDF paper, DZero paper).

The CDF detector at Fermilab

The CDF detector at Fermilab weighs about 6000 tons. It records the particles emerging from high-energy proton-antiproton collisions produced by the Tevatron.

A couple of years ago, the two collaborations had reported preliminary results on the search for the single top.  Since then, experimenters have more than doubled the amount of data analyzed and sharpened selection and analysis techniques, making the discovery possible. For each experiment, the probability that background events have faked the signal is now only one in nearly four million, allowing both collaborations to claim a bona fide discovery that paves the way to more discoveries.

"I am thrilled that CDF and DZero achieved this goal," says Fermilab Director Pier Oddone. "The two collaborations have been searching for this rare process for the last fifteen years, starting before the discovery of the top quark in 1995. Investigating these subatomic processes in more detail may open a window onto physics phenomena beyond the Standard Model."

The Standard Model of particles and their interactions is one of the most successful theoretical frameworks in physics, yet scientists know that it only explains a fraction of all matter and energy in the universe. Dark matter and dark energy, which the Standard Model does not explain, make up about 95 percent of the universe.