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Physicists get to heart of antimatter

Like Jekyll and Hyde, some subatomic particles are able to act as both matter and their antimatter counterparts. Known as mixing, this process has been known to quantum physicists for 50 years. Now it has been measured for the first time by an international collaboration involving MIT scientists.

The work could lead to a better understanding of the early universe, when these particles were present in great abundance.

The achievement was announced yesterday by Ivan Furic (MIT Ph.D. 2004), now at the University of Chicago, representing the Collider Detector at Fermilab (CDF) collaboration at the Fermi National Accelerator Laboratory.

The CDF team specifically reported rapid-fire transitions between matter and antimatter of a subatomic particle called the Bs (pronounced "B sub s") meson. They found that this particle oscillates between matter and antimatter states at a mind-boggling 3 trillion times per second.

The Bs itself is composed of other subatomic particles: a heavy "bottom quark" bound to a "strange anti-quark."

Christoph Paus, associate professor of physics (and Furic's thesis advisor at MIT), represents MIT in the CDF collaboration, a team of 700 physicists from 61 institutions and 13 countries. Paus, a member of MIT's Laboratory for Nuclear Science, led the data analysis effort involving 80 scientists from 27 institutions.

The result was announced just one month after the data-taking was completed at the CDF, the world's highest-energy particle accelerator.

"The rapid matter-antimatter oscillations, 3 trillion times per second, give us a glimpse at the development of the early universe and might help us understand why there is so little antimatter in it right now," Paus said.

The researchers' goals are to discover the identity and properties of the particles that make up the universe and to understand the forces and interactions between those particles. Over the past 20 years, many experiments worldwide have been part of a program to make high-precision measurements of the behavior of matter and antimatter.

Scientists hope that by assembling a large number of precise measurements involving the exotic behavior of these particles, they can begin to understand why these particles exist, how they interact with one another and what role they played in the development of the early universe.

Although none of these particles exists in nature today, they were present in great abundance in the early universe. Scientists can only study these particles now at large particle accelerators.

Within the high-energy physics community, the new measurement will immediately be interpreted within different theoretical models. The fact that it confirms the 25-year-old existing theory, the Standard Model of Particle Physics, "means that nature has not yet revealed its secret for why matter, and not antimatter, dominates the universe," Paus said, "although this result will refute some theories based on even faster oscillations.

"As an experimentalist, the pleasure of ruling out new theories is second only to ruling out the existing one," Paus said. "This measurement is not the end of the story. It opens new venues to pursue the quest for nature's best-kept secrets."

CDF is supported by the DOE, the NSF and international funding agencies.

A version of this article appeared in MIT Tech Talk on April 12, 2006 (download PDF).

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