Apr 13 2006
Scientists at the Department of Energy's Fermi National Accelerator Laboratory have announced the precision measurement of extremely rapid transitions between matter and antimatter. The observations were made at the Collider Detector at Fermilab, representing a massive collaborative effort involving 700 researchers from 61 institutions, including the University of Pennsylvania.
It has been known for 50 years that very special species of subatomic particles can make spontaneous transitions between matter and antimatter. In this new result, CDF physicists measured the rate of the matter-antimatter transitions for the Bs (pronounced "B sub s") meson, which consists of the heavy bottom quark bound by the strong nuclear interaction to a strange anti-quark, a staggering rate: 3 trillion times per second. They were able to detect this oscillation occur four times over the one and a half picoseconds that the Bs meson took to fly through the collider.
"The measurement of this frequency of particle-antiparticle transformations allows us to measure fundamental parameters in our standard model of particle physics," Joseph Kroll, associate professor in Penn Department of Physics and Astronomy, said. "In addition, it is a first step in making many more measurements with the Bs meson. These measurements may shed light on the matter-antimatter asymmetry in the universe and may provide unambiguous evidence for the existence of new particles not currently contained in our standard model."
Kroll spearheaded a component of the CDF experiment, the Time-of-Flight detector, which was crucial for making it possible to observe the oscillations. He and his Penn colleagues were responsible for designing and building all the electronics in the signal path for this detector, and they play a major role in overseeing the entire project.
During the last 20 years, a large number of experiments worldwide have participated in a program to perform high precision measurements of the behavior of matter and antimatter, especially as it pertains to strange, charm and bottom quarks. The physics of particles containing bottom quarks prompted the construction of two accelerator complexes, one in Stanford, Calif., and the other in Tsukuba, Japan, to study these particles.
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. Although none of them exists in nature today, these particles were present in great abundance in the early universe. Scientists can only study them at large particle accelerators.
With a talk at Fermilab on Monday afternoon, the CDF collaboration presented to the scientific community the first measurement of this Bs matter-antimatter transition rate of about 3 trillion times per second, measured to a precision of 2 percent. They reported on data acquired by the CDF detector between February 2002 and January 2006, a running period known as "Tevatron Run II," where tens of trillions of proton-antiproton collisions were produced at the Tevatron. There have been many attempts to measure this rate. The most recent result comes from the D0 collaboration (CDF's sister experiment at the Tevatron) where they announced upper and lower bounds on the oscillation frequency.
"Exploration of the anti-world's mysteries is a crucial step towards our understanding of the early universe, and how we came to be," Raymond Orbach, director of the DOE Office of Science, said. "Discoveries as important as oscillations to and from the antiworld have been made possible by the remarkable, record-breaking Run II luminosity of the Tevatron, a tribute to the skill of the Fermilab family."
"Many of the upgrades to the CDF detector for Run II were aimed at increasing our sensitivity to observing Bs oscillations," Penn's Kroll said. "Every collaborator contributed in some way to this measurement. It is very exciting to finally achieve this goal."
Kroll was co-leader to the team of 80 scientists from 27 institutions performed the data analysis leading to the precision measurement just one month after the data-taking was completed.
Within the high energy physics community, this CDF precision measurement will immediately be interpreted within different theoretical models of how the universe is assembled. One popular and well motivated theory is supersymmetry, in which each known particle has its own "super" partner particle. Fermilab theoretical physicist Marcela Carena noted that general versions of supersymmetry predict an even faster transition rate than was actually measured, so some of those theories can be ruled out based upon this result.
"At the Tevatron," Carena said, "important information on the nature of supersymmetric models will be obtained from the combination of precise measurements of Bs matter-antimatter transitions and the search for the rare decay of Bs mesons into muon pairs. It is even possible that an indirect indication for supersymmetry would show up in these measurements before the Large Hadron Collider turns on at CERN."
Both D0 and CDF experiments expect to achieve improved results in these areas in the near future.
This research was supported by grants from the Department of Energy and the National Science Foundation.
Additional information on the announcement of this discovery and a full list of collaborators is available at www.fnal.gov.
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