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Single Top Quarks Observed

Research Team Co-Led by UCR Physicist Observes Production of Single Top Quarks, Fundamental Constituents of Matter

Top quark provides clues to solving long-standing mysteries about the universe

(March 9, 2009)

Ann Heinson is a research physicist in UCR's Department of Physics and Astronomy.  Photo credit: J. Ellison, UC Riverside.Enlarge

Ann Heinson is a research physicist in UCR's Department of Physics and Astronomy. Photo credit: J. Ellison, UC Riverside.

RIVERSIDE, Calif. – A group of 28 scientists at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, co-led by UC Riverside’s Ann Heinson, has made the first observation of the production of single top quarks – an observation that resulted from proton-antiproton collisions measured by the DZero detector in Fermilab’s Tevatron, the world’s highest-energy particle collider.

Previously, top quarks had only been observed when produced by the strong interaction between tiny elementary particles. That process 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 production is a time-consuming business. Only one in every 20 billion proton-antiproton collisions produces a single top quark. Moreover, the signal of these rare occurrences is easily mimicked by other “background” processes that occur at much higher rates.

The heaviest known elementary particle, the top quark is nearly 200 times heavier than a proton in an atom’s nucleus. Point-like, it has no size. It has the same mass as a gold atom and is one of the fundamental building blocks of nature.

Understood as an ingredient of the particle soup just after the Big Bang, today the top quark does not occur naturally but must be created experimentally in a high-energy particle accelerator, such as the Tevatron, that can recreate the conditions of the early universe.

“Two years ago, we found the first evidence for single-top-quark production, but now we have more than doubled the dataset and improved our selection and analysis techniques,” said Heinson, a research physicist in the Department of Physics and Astronomy and a fellow of the American Physical Society. “The probability for the background to fake the signal is one in four million. We can confidently claim, therefore, that this is the first observation of single-top-quark production.”

To make the single-top discovery, Heinson’s team of researchers, who are all members of the “DZero collaboration,” spent two years combing through the results of proton-antiproton collisions recorded by the DZero experiment. The DZero collaboration is an international team of nearly 500 scientists studying the top quark in particle collisions.

Heinson’s team identified several thousand collision events that looked the way single top events are expected to appear. Using sophisticated statistical analysis and detailed background modeling, the team showed that a few hundred collision events produced the real thing.

The DZero collaboration submitted its results to Physical Review Letters on March 4.

“We plan to use these collision events to measure several parameters of the Standard Model and to open a window onto many different processes that could exist beyond the Standard Model,” Heinson said.

High-energy physics focuses on what makes up the world, and what holds it together. Its Standard Model is a comprehensive theory that explains the interactions between all fundamental elementary particles. The top quark is the Standard Model’s heaviest particle.

Having more precise information about the top quark gives scientists clues in their search for another missing puzzle piece, the Higgs boson, nicknamed “the God particle,” which many physicists believe will solve long-standing mysteries about the universe.

“We would not be able to claim evidence for Higgs production in the future if we did not first observe single top quark production,” Heinson explained. “The top quark is easier to find, and it forms part of the background to Higgs signals. Now, with a large sample of single top quark events in hand, we will be able to use the events to probe many aspects of the Standard Model.”

The 28-member research team co-led by Heinson includes UCR’s Liang Li, a postdoctoral researcher, and Mark Padilla, a graduate student.

Major funding for the study was provided by the U.S. Department of Energy.

The University of California, Riverside ( is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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