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Author Topic: Meson that switches between matter and antimatter  (Read 2088 times)


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Fermilab's CDF scientists make it official: They have discovered the quick-change behavior of the B-sub-s meson, which switches between matter and antimatter 3 trillion times a second.

BATAVIA, Illinois - Scientists of the CDF collaboration at the Department of Energy's Fermi National Accelerator Laboratory announced today (September 25, 2006) that they have met the exacting standard to claim discovery of astonishingly rapid transitions between matter and antimatter: 3 trillion oscillations per second.

Dr. Raymond L. Orbach, Undersecretary for Science in the U.S. Department of Energy, congratulated the CDF collaboration on the result.

"This remarkable tour de force details with exquisite precision how the antiworld is tied to our everyday realm," Dr. Orbach said. "It is a beautiful example of how, using increasingly sophisticated analysis, one can extract discovery from data from which much less was expected. It is a triumph for Fermilab."

The CDF discovery of the oscillation rate, marking the final chapter in a 20-year search, is immediately significant for two major reasons: reinforcing the validity of the Standard Model, which governs physicists' understanding of the fundamental particles and forces; and narrowing down the possible forms of supersymmetry, a theory proposing that each known particle has its own more massive "super" partner particle.

Many experiments worldwide have worked to perform high precision measurements of the behavior of matter and antimatter, especially as it pertains to strange, charm and bottom quarks. Scientists hope that by assembling a large number of precise measurements involving the exotic behavior of these particles, they can begin to understand why they exist, how they interact with one another and what role they played in the development of the early universe. Most importantly, they could also be the place in which to look for new physics beyond the Standard Model, which scientists believe is incomplete. Although none of these particles exists in nature today, they were, however, present in great abundance in the early universe. Thus, scientists can only produce and study them at large particle accelerators.

With a talk at Fermilab on Friday, September 22, Christoph Paus of the Massachusetts Institute of Technology, representing the CDF experiment, presented the discovery to the scientific community. The experimenters acquired their data between February 2002 and January 2006, an operating period known as Tevatron Run 2, where tens of trillions of proton-antiproton collisions were produced at the world's highest-energy particle accelerator. The results have been submitted in a paper to Physical Review Letters.

This first major discovery of Run 2 continues the tradition of particle physics discoveries at Fermilab, where the bottom (1977) and top (1995) quarks were discovered. Surprisingly, the bizarre behavior of the B_s (pronounced "B sub s") mesons is actually predicted by the Standard Model of fundamental particles and forces. The discovery of this oscillatory behavior is thus another reinforcement of the Standard Model's durability.

"Scientists have been pursuing this measurement for two decades, but the convergence of capabilities to make it possible has occurred just now," said CDF cospokesperson Jacobo Konigsberg of the University of Florida. "We needed to produce sufficient quantities to be able to study these particles in detail. That condition was met by the superb performance of the Tevatron. Then, with a process this fast, we needed extremely precise detectors and sophisticated analysis tools. Those conditions were met at CDF, along with the skill and contributions of a great team of people."

CDF physicists have previously measured the rate of the matter-antimatter transitions for the B_s meson, which consists of the heavy bottom quark bound by the strong nuclear interaction to a strange antiquark. Now they have achieved the standard for a discovery in the field of particle physics, where the probability for a false observation must be proven to be less than about 5 in 10 million (5/10,000,000). For CDF's result the probability is even smaller, at 8 in 100 million (8/100,000,000).

"Everyone in Fermilab's Accelerator Division has worked hard to create the number of collisions that were required to reach this impressive result," said Fermilab Director Pier Oddone. "We're glad that CDF has been able to put these efforts to such good effect. This is one of the signature measurements for Run II, and as we collect several times the data already on hand, I have great expectations for future discoveries."

Determining the astonishing rate of 3 trillion oscillations per second required sophisticated analysis techniques. CDF cospokespersons Konigsberg and Fermilab's Rob Roser explained that the B_s meson is a very short-lived particle. In order to understand its underlying characteristics, scientists have to observe how each particle decays to determine its true make-up.

"Developing the software tools to make maximal use of the information in each collision takes time and effort," said Roser, "but the rewards are there in terms of discovery potential and increased level of precision."

Many different theoretical models of how the universe works at a fundamental level will be now be confronted with the CDF discovery. The currently popular models of supersymmetry, for example, predict a much higher transition frequency than that observed by CDF, and those models will need to be reconsidered.

Marvin Goldberg, Division of Physics program director of the National Science Foundation, emphasized the collaborative role of the experimenters.

"This result reminds us that discoveries in particle physics require the coherent efforts of many people as well as advanced physical infrastructure," Goldberg said. "By combining the luminosity of the Tevatron, the precision of the CDF detector and the intellectual prowess of the international CDF collaboration with sophisticated data analysis, this remarkable result from a remarkable effort will advance our understanding of the way the universe works."

To further advance that understanding, Roser, Konigsberg and their colleagues continue to seek phenomena that are not predicted by the Standard Model. The prize would be a discovery of new physics.

"While the B_s oscillation discovery was one of the benchmark results that we wanted from the Tevatron," said Roser, "we still have more than half the data from Run 2 waiting to be analyzed. We're looking forward to more results, and we're always hoping for surprises."

CDF is an international experiment of 700 physicists from 61 institutions and 13 countries. It is supported by DOE, NSF and a number of international funding agencies (the full list can be found at With the Tevatron, the world's highest-energy particle accelerator, in 1995 the CDF and DZero collaborations discovered the top quark, the final and most massive quark in the Standard Model.

Fermilab is a Department of Energy Office of Science national laboratory operated under contract by Universities Research Association, Inc.

Graphic: The figure shows the CDF measurement of the B_s oscillation frequency at 2.8 trillion times per second. The analysis is designed such that possible oscillation frequencies have an amplitude consistent with 1.0 while those not present in the data will have an amplitude consistent with zero. Image courtesy CDF collaboration.

« Last Edit: 26/09/2006 21:24:45 by another_someone »


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