University of Pennsylvania Scientists Fundamental to the Discovery of Exotic Matter-Antimatter Particle

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Media Contact:Greg Lester | | 215-573-6604October 19, 2006

Scientists at the Department of Energy's Fermi National Accelerator Laboratory announced Sept. 25 that they have met the standard to claim discovery of astonishingly rapid transitions between matter and antimatter: 3 trillion oscillations per second.  Their findings confirm the rate of matter-antimatter transitions for the Bs (pronounced "B sub s") meson first described last April.

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.  Their observations have been submitted to the journal Physical Review Letters.

"The measurement of this frequency of particle-antiparticle transformations allows us to measure fundamental parameters in our Standard Model of particle physics," said Joseph Kroll, professor in Penns Department of Physics and Astronomy and a lead author of the findings.  "This measurement can be used to shed light on the existence of new particles, such as supersymmetric particles, not currently contained in our standard model."

Kroll spearheaded the proposal of one component of the CDF experiment, the Time-of-Flight detector, which was crucial for making it possible to observe the oscillations.   With funding from Penn and the Department of Energy, Kroll and colleagues were responsible for designing and building all the electronics in the signal path for this detector, and they played a major role in overseeing the entire project.  

Particle physicist Paul Langacker, who was chair of the Department of Physics and Astronomy when Kroll joined the project in the late 1990s, described the findings "as subtle and beautiful measurements that confirmed the Standard Theory of weak interactions and constrained the possibilities of new particles and interactions."

The CDF physicists measured the rate of the matter-antimatter transitions for the Bs meson, which consists of the heavy bottom quark bound by the strong nuclear interaction to a strange anti-quark.  They were able to detect this oscillation occur four times during the one-and-a-half picoseconds that the Bs meson took to fly through the collider.  The result reported in April was convincing: the probability that a statistical fluctuation could produce the signal that was observed then was about 0.2%.

This probability, however, does not meet the exacting standard of definite discovery, which is 6 in 10 million.   So the CDF physicists improved their data-analysis techniques.  The probability that a random fluctuation would produce a signal comparable to this new signal is 8 in 100 million, well below the accepted standard.  This final measurement marks the end of almost 20 years of effort by many experiments and several international accelerator facilities to measure this oscillation frequency.

"For the past decade, I have been focused on this measurement; it is very satisfying to finally finish this chapter in particle physics," Kroll said.

"The success of the Fermilabs progam that just announced precision measurements of the oscillations of B_s mesons is due in no small part to the leadership, dedication and long hours of work of Joe Kroll and the Penn CDF team," said Thomas Lubensky, professor and chair of Penns Department of Physics and Astronomy.

Additional information and a list of collaborators is available  at