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Fermilab collider experiments discover rare single top quark

News Release — 10 March 2009


Media contact: Sue Stevens, Senior Media Relations Officer, 817- 272-2761,

ARLINGTON - Scientists at the Department of Energy's Fermi National Accelerator Laboratory in Batavia, Ill., have observed particle collisions that produce single-top quarks. The discovery is important because it confirms important parameters of particle physics, including the total number of quarks, and has significance for the ongoing search for the Higgs particle at Fermilab's Tevatron, currently the world's most powerful operating particle accelerator. The Higgs boson is the only Standard Model particle that has not yet been observed. Experimental detection of the Higgs boson would help explain how massless elementary particles can have mass.

The University of Texas at Arlington's High Energy Physics Group has worked in a collaboration at Fermilab for 18 years. Participants are physics professors Kaushik De, Andrew White, Andrew Brandt and Jaehoon Yu.

Previously, top quarks had been observed only 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 is harder to identify experimentally, has now been observed, almost 14 years to the day of the top quark discovery in 1995.

Searching for single top quark 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.

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. In addition, the single top and the Higgs signals have backgrounds in common, and the single top is itself a background for the Higgs particle.

To make the single top discovery, physicists in two Fermilab collaborations spent years combing through the results of proton-antiproton collisions recorded by their experiments 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.

The collaborations had reported preliminary results on the search for the single top earlier.  Since then, researchers 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.



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