20 Year Long Top Quark Puzzle Completed
    By News Staff | February 24th 2014 03:40 PM | 4 comments | Print | E-mail | Track Comments

    The Fermi National Accelerator Laboratory is still getting important particle work done, years after the closure of the Tevatron was announced.

    Scientists on the CDF and DZero
    experiments have announced that they have found the final predicted way of creating a top quark, completing a picture of this particle that has been nearly 20 years in the making.

    The collaborations announced late last week that they had observed one of the rarest methods of producing the elementary particle – creating a single top quark through the weak nuclear force using the "s-channel." For the analysis, they sifted through data from more than 500 trillion proton-antiproton collisions produced by the Tevatron from 2001 to 2011 and identified about 40 particle collisions in which the weak nuclear force produced single top quarks in conjunction with single bottom quarks.

    Top quarks are the heaviest and among the most puzzling elementary particles. They weigh even more than the Higgs boson – as much as an atom of gold – and only two machines have ever produced them: Fermilab's Tevatron and the Large Hadron Collider at CERN. There are several ways to produce them, as predicted by the theoretical framework known as the Standard Model, and the most common one was the first one discovered: a collision in which the strong nuclear force creates a pair consisting of a top quark and its antimatter cousin, the anti-top quark.

    The process for creating single top quarks through the s-channel. A quark from an incoming proton interacts in the Tevatron with an antiquark from an incoming antiproton, forming a W boson with much greater mass. This W boson then decays into a top quark and an antibottom quark, which can be seen in the CDF and DZero detectors. Credit: Fermilab

    Collisions that produce a single top quark through the weak nuclear force are rarer, and the process scientists on the Tevatron experiments have just announced is the most challenging of these to detect. This method of producing single top quarks is among the rarest interactions allowed by the laws of physics. The detection of this process was one of the ultimate goals of the Tevatron, which for 25 years was the most powerful particle collider in the world.

    "This is an important discovery that provides a valuable addition to the picture of the Standard Model universe," said James Siegrist, Department Of Energy (DOE) Associate Director of Science for High Energy Physics. "It completes a portrait of one of the fundamental particles of our universe, by showing us one of the rarest ways to create them."

    Searching for single top quarks is like looking for a needle in billions of haystacks. Only one in every 50 billion Tevatron collisions produced a single s-channel top quark, and the CDF and DZero collaborations only selected a small fraction of those to separate them from background, which is why the number of observed occurrences of this particular channel is so small. However, the statistical significance of the CDF and DZero data exceeds that required to claim a discovery.

    "Kudos to the CDF and DZero collaborations for their work in discovering this process," said Saul Gonzalez, program director for the National Science Foundation. "Researchers from around the world, including dozens of universities in the United States, contributed to this important find."

    The CDF and DZero experiments first observed particle collisions that created single top quarks through a different process of the weak nuclear force in 2009. This observation was later confirmed by scientists using the Large Hadron Collider. 

    "I'm pleased that the CDF and DZero collaborations have brought their study of the top quark full circle," said Fermilab Director Nigel Lockyer. "The legacy of the Tevatron is indelible, and this discovery only makes the breadth of that research even more remarkable."


    The first sentence of this post is incorrect and repeats a common misconception. The fact is that Fermi National Accelerator Laboratory, also known as Fermilab, is not closing, and nobody has ever announced the closure of the laboratory. In 2010, the laboratory announced that it would shut down its particle collider, the Tevatron, which happened as planned in 2011. But the laboratory continues to operate seven other particles accelerators and storage rings, and it is constructing several new particle physics experiments. One of them, the NOvA neutrino experiment, just observed the first neutrinos produced by accelerators at Fermilab:

    In addition, the laboratory has an active accelerator R&D program, contributes expertise and computing power to the CMS experiment at the Large Hadron Collider, and has an active particle astrophysics program, including experiments that search for dark-matter particles and the origin of dark energy. To learn more about the laboratory, visit

    Did they edit the article?
    The first sentence now says "closure of the Tevatron" and doesn't say Fermilab shut down.
    Or maybe you misread it?

    Yes, we got an email from Kurt and corrected the error, but it did say the lab rather than just the Tevatron originally.
    Thanks for fixing the first sentence about Fermilab. It's now correct.