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BABAR Discovers New Difference Between Matter and Antimatter

News Release | For Immediate Release | July 06, 2001

(Vancouver, BC) -- An international scientific collaboration, including physicists from the University of British Columbia, University of Victoria, the Université de Montréal and McGill University, have discovered a new fundamental difference between matter and antimatter. The team, working on the detector known as BABAR at the Stanford Linear Accelerator Center (SLAC) in California, has observed the intriguing effect known as CP-violation in the disintegrations of heavy, short-lived subatomic particles called B mesons.

This subtle difference between matter and antimatter was first seen more than 35 years ago in experiments with another particle, called the kaon. Now for the first time BABAR has shown that another type of particle exhibits this puzzling phenomenon, which should help physicists to understand the domination of matter over antimatter in the Universe. It is important, because unlike in kaon experiments, the BABAR physicists have been able to measure directly a parameter in the Standard Model of particle physics.

'This result determines directly the magnitude of the fundamental matter/anti-matter asymmetry in Nature.' says Christopher Hearty, from the University of British Columbia.

BABAR is a sophisticated 1,200-tonne detector, built and operated by a team of more than 600 physicists and engineers from nine countries**. A central component of the detector, the Drift Chamber, was constructed at TRIUMF, Canada's national laboratory for subatomic physics, which is funded by the National Research Council. BABAR records subtle distinctions between decays of B mesons and those of their antimatter counterparts, called anti-B mesons. Both are more than five times as heavy as the more familiar proton and survive just over a trillionth of a second. The physicists used the detector to observe an unmistakable difference, or asymmetry, between the rates at which B and anti-B mesons decay into a special set of other, lighter particles. From these measurements, they calculate a parameter called 'sine two-beta', which expresses the degree of asymmetry between matter and antimatter. A non-zero value of this parameter is clear evidence for CP violation among B mesons.

The collaboration has reported its result in a paper submitted July 5 for publication in the leading scientific journal Physical Review Letters. In the paper, the team reports measuring a value of sine two-beta = 0.59 +/- 0.14, which is substantially different from zero. There are now less than 3 chances in 100,000 that the actual, physical asymmetry could be consistent with zero.

This result from BABAR is easily the most precise measurement of sine two-beta reported to date. Earlier measurements made at Fermi National Accelerator Laboratory, the Japanese National Laboratory for High-Energy Accelerator Research (KEK), and by the BABAR team are consistent with the present result, but they are not as accurate.

'We are now poised for further discoveries that should open up new directions for particle physics', says Stewart Smith, from Princeton University and Spokesman for the BABAR collaboration.

The precision of the BABAR result was made possible by the outstanding performance of the PEP-II 'B Factory' at SLAC. Built in collaboration with the Lawrence Berkeley and Lawrence Livermore National Laboratories, this pair of 2.2 kilometer storage rings collides unequal-energy beams of electrons and their antimatter counterparts, called positrons. Since it began operating in June 1999, the B Factory has produced more than 32 million pairs of B mesons, from which data the present BABAR result was extracted.

'The B Factory has performed beyond expectations, permitting the BABAR collaboration to make the world-class measurements on B mesons,' says SLAC Director Jonathan Dorfan, who played a pivotal role in designing and building the particle collider.


TRIUMF is Canada's National Laboratory for Particle and Nuclear Physics. Located on the south campus of the University of British Columbia, TRIUMF is owned and operated as a joint venture by a consortium of the following Canadian universities, via a contribution through the National Research Council Canada: University of Alberta, University of British Columbia, University of Calgary, Carleton University, University of Guelph, University of Manitoba, McMaster University, Université de Montréal, Queen's University, University of Regina, Simon Fraser University, Saint Mary's University, University of Toronto, University of Victoria, York University.


Photographs and illustrations of the B Factory and BABAR detector can be found at:

** The BABAR collaboration includes physicists from 73 institutions in Canada, China, France, Germany, Great Britain, Italy, Norway, Russia and the United States. The Canadian BABAR group, which is supported by the Natural Sciences and Engineering Research Council of Canada, consists of:
Christopher Hearty, Janis A. McKenna and Tom S. Mattison, University of British Columbia
Robert Kowalewski and J. Michael Roney, University of Victoria
Paul Taras, Université de Montréal
Popat M. Patel, McGill University

*** The Stanford Linear Accelerator Center is a national laboratory for high-energy physics and synchrotron-radiation research operated by Stanford University on behalf of the U.S. Department of Energy. The Department's Office of Science funded the construction of the B Factory at $177 million and contributed about 60 percent of the cost of the BABAR detector, with the remainder coming from foreign sources.

'Foreign contributions to this experiment, both monetary and scientific, have been absolutely crucial to its success,' notes David Hitlin of the California Institute of Technology, who assembled the collaboration and oversaw the construction of the detector.


The mysterious phenomenon of CP violation was first discovered in a 1964 experiment at the Brookhaven National Laboratory led by James Cronin and Val Fitch. Their group observed this behavior in decays of subatomic particles called K mesons, which are about one tenth as heavy as B mesons and live much longer; the two physicists shared a Nobel prize for the discovery. Since then, several observations of CP violation have occurred in experiments with K mesons, but until the recent BaBar discovery, no other subatomic particles had clearly exhibited this exceedingly rare phenomenon. Having a second striking example of CP violation should aid theorists who are trying to understand what causes it.

Scientists are extremely interested in this puzzling behavior because it may help explain why there is such a preponderance of matter in the Universe. In 1967, Russian theorist Andrei Sakharov used CP violation to suggest how the present matter-dominated Universe could have emerged from one that contained exactly equal amounts of matter and antimatter during the earliest moments of the Big Bang.

Further background information can be found at:

For additional information, please contact:

Dr. Chris Hearty
University of British Columbia


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