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June 22, 2006 - Physicists Size Up the 'Unitarity Triangle' - Press Release

Date Issued: June 22, 2006


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The Unitarity Triangle. Graphic source: CKMfitter collaboration. (Click on image for larger version.)

Menlo Park, CA—B factory experiments at the Department of Energy's Stanford Linear Accelerator Center (SLAC) and at the High Energy Accelerator Research Organization (KEK) in Japan have reached a new milestone in the quest to understand the matter-antimatter imbalance in our universe.

Experimenters have leaped from inference to direct knowledge of the proportions of the B unitarity triangle. Not just a simple geometric shape, this triangle summarizes knowledge of the rare processes that contribute to the universe's partiality for matter over antimatter.

The area of the triangle visually depicts the amount of difference, or asymmetry, between the decays of B particles and their antimatter counterparts, anti-B particles.

Thanks to the accumulation of hundreds of millions of B and anti-B particles produced at the two laboratories, scientists have been able to measure all three angles of the triangle from measurements of matter-antimatter differences.

"Based on these asymmetry measurements alone, we now know for the first time that the B unitarity triangle really does have finite area," said David MacFarlane, spokesperson for the BaBar experiment at SLAC.

This is an important jump forward because until now physicists have relied on measurements of the sides of the triangle. Proving that the sides really form a triangle requires the matter-antimatter measurements.

The direct measurement of the unitarity triangle's angles generates an area that is consistent with the area predicted by measurements of the sides.

"Such a confrontation between prediction and direct measurement is the very essence of science and has been a major goal for the two experiments,” MacFarlane said. "Once again, we have seen the power of precision measurements to peer into the future and infer solutions that could not have been experimentally determined at the time."

A number of measurements made over the past 50 years painted an increasingly precise picture of what the unitarity triangle should look like. Once the B factories had accumulated enough data, physicists could satisfy their hunger to know if the inferred size and shape of the triangle held up. In other words, did they really understand the unitarity triangle and what it said about the origin of the asymmetry between matter and antimatter?

The answer is yes: the new triangle matches the indirectly pieced-together knowledge of the triangle. Drawing the triangle directly, by using only measurements of matter-antimatter asymmetry in B decays, confirms the Standard Model, which predicts rates of particle decays.

"It's a real milestone and an elegant culmination of a 50-year investigation across an array of very different experiments," said Steve Olsen, co-spokesperson for the Belle experiment in Japan.

Taken together, the three angles of the triangle are now known with enough precision for physicists to confidently pin down the triangle's area. It's an outstanding feat: the asymmetry in B particle decays was discovered only five years ago and now physicists have made enough measurements to determine the angle called beta to better than 5 percent precision.

To measure the angle alpha with much greater accuracy than previously possible, the BaBar team found a way to use a particular decay mode that initially was thought to be too difficult to measure. The alpha angle is currently measured with a 15 percent precision.

"This has turned out to be the best approach, taking errors down three-fold from a year ago," said MacFarlane.

An innovative analysis approach introduced by Belle experimenters opened up the possibility for measuring gamma as well, saving the B factory experiments from many years of additional data accumulation. Although gamma is the least-known angle, physicists have achieved enough precision to verify a closed triangle.

The outstanding agreement between the asymmetry measurements and the knowledge of the triangle's sides still leaves researchers with a real puzzle. The amount of asymmetry found experimentally is still far too small to explain why we live in a universe of matter rather than antimatter. It may take new kinds of physics to explain the missing antimatter. A much deeper understanding of nature and matter-antimatter asymmetry is expected with further studies of B mesons. The LHCB experiment at CERN in Geneva will start taking data in 2008 and scientists are looking into the possibility of an electron-positron Super B factory with 100 times higher performance than the current experiments.


Some 600 scientists and engineers from 75 institutions in Canada, China, France, Germany, Italy, the Netherlands, Norway, Russia, Spain the United Kingdom, and the United States are working on BaBar. SLAC is funded by the Department of Energy's Office of Science.

The Belle experiment in Tsukuba, Japan consists of about 400 physicists from 55 institutions in Australia, Austria, China, Germany, India, Japan, Korea, Poland, Russia, Slovenia, Swiss, Taiwan and the United States. KEK is funded by Ministry of Education, Culture, Sports, Science and Technology in Japan.

by Heather Rock Woods


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