March 10, 2009

Fermilab Scientists Discover Rare Single Top Quark

A group of 28 scientists at the U.S. Department of Energy's Fermi National Accelerator Laboratory (Fermilab) have observed particle collisions that produce single top quarks.

The observation resulted from proton-antiproton collisions measured by the DZero detector in Fermilab's Tevatron, currently the world's most powerful operating particle accelerator.

The discovery of the single top quark confirms critical parameters of particle physics, including the total number of quarks, and is important for the ongoing search for the Higgs particle at Fermilab's Tevatron.

Previously, top quarks had only been observed when produced by 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 more difficult to identify experimentally, has now been observed.  The finding comes almost 14 years to the day after the top quark discovery in 1995.

Searching for single-top production makes finding a needle in a haystack look simple.  Indeed, only one in every 20-billion proton-antiproton collisions produces a single top quark.   Furthermore, 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," said Dr. Dennis Kovar, Associate Director of the Office of Science for High Energy Physics at the U.S. Department of Energy, in a press release issued by Fermilab.

"Furthermore, 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. 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 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. Complex  statistical analysis and detailed background modeling showed that a few hundred collision events produced the real thing. 

The two collaborations had previously reported initial 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 true discovery.

"I am thrilled that CDF and DZero achieved this goal," said 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."

CDF is an international experiment of 635 physicists from 63 institutions in 15 countries. DZero is an international experiment conducted by 600 physicists from 90 institutions in 18 countries. Funding for the CDF and DZero experiments comes from DOE's Office of Science, the National Science Foundation, and international funding agencies.

The two teams submitted their independent results to Physical Review Letters on March 4, 2009.


Image 1: The Fermilab accelerator complex accelerates protons and antiprotons close to the speed of light. The Tevatron collider, four miles in circumference, produces millions of proton-antiproton collisions per second, maximizing the chance for discovery. Two experiments, CDF and DZero, record the collisions to look for signs of new particles and subatomic processes.

Image 2: The DZero detector records particles emerging from high-energy proton-antiproton collisions produced by the Tevatron. Tracing the particles back to the center of the collision, scientists understand the subatomic processes that take place at the core of proton-antiproton collisions. Scientists search for the tiny fraction of collisions that might have produced single top quarks.


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