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Stanford Report, August 9, 2000

Multinational experiments offer clues to lopsided universe

BY DAWN LEVY AND MICHAEL RIORDAN

A perfectly symmetrical universe would contain nothing. Zip. Nada. The energy of the Big Bang created equal amounts of matter and antimatter, which should have annihilated each other completely (see accompanying story). Since that clearly is not the case -- matter abounds while antimatter is scarce -- there must be some subtle asymmetry between the two. To understand that asymmetry, physicists at the Stanford Linear Accelerator Center and elsewhere have been studying the behavior of the smallest of particles to answer the biggest of questions: Where did everything come from?


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On July 31, scientists from SLAC and Japan's KEK (the Japanese acronym for High Energy Accelerator Research Organization) presented the first results of two large, multinational experiments aimed at explaining why the universe contains much more matter than antimatter. At the International Conference on High Energy Physics in Osaka, Japan, they reported hints of asymmetry -- called charge-parity or CP violation -- in the decays of subatomic particles called B mesons. (Charge refers to a particle's positive or negative electrical property, and parity refers to its right-handed or left-handed "spin.")

Positrons and electrons are brought together for mutual destruction inside large particle detectors such as SLAC's BABAR.

"Our results are very preliminary," says physicist David Hitlin of Caltech, spokesman of the SLAC-based project. But if they hold up as more data pour in, he says, that would be "very exciting," since theorists would have to reconcile the amount of observed asymmetry with what is predicted by the Standard Model of particle physics.

More than 500 physicists from nine countries participated in the SLAC-based BABAR collaboration sponsored by the U.S. Department of Energy (DOE). (BABAR is a play on the name of a B-meson antimatter particle, called "B-bar," and the children's book elephant that is the project's mascot.) About 300 international scientists took part in the KEK experiment, nicknamed Belle.

To study the behavior of B mesons, scientists at so-called "B factories" produce millions of B mesons and their antimatter counterparts. With a precision that was never before possible, they are able to compare particle and antiparticle decay rates to measure the degree of symmetry.

At the B factory, electrons and positrons circulate at different energies in two rings. They meet at a point and annihilate. The resulting B meson and its antiparticle continue in the direction of the more energetic beam, making it easier to isolate and characterize daughter particles arising from B decays.

Designed and built by SLAC in cooperation with the Lawrence Berkeley and Lawrence Livermore National Laboratories with $177 million in DOE funding, Stanford's B Factory is an innovative electron-positron collider creating millions of short-lived B mesons. Their behavior provides clues that will help collaborators from Canada, China, France, Germany, Great Britain, Italy, Norway, Russia and all across the United States search for subtle differences between matter and antimatter.

So far, of 9 million B-meson pairs made at Stanford's B Factory, 120 have been of a special type that allows scientists to look for CP violation. The KEK experimenters have accumulated less data than did the BABAR scientists but found a greater degree of asymmetry that is in agreement with the Standard Model. But errors are large in both cases, so scientists need to continue to collect data to provide stronger evidence of CP violation.

The B Factory has exceeded expectations and continues to do so, having nearly reached design performance after only a year of operation, according to SLAC Director Jonathan Dorfan. Normally, a new particle collider with such innovative features must undergo a long tuning period, often taking several years before it performs at its full potential. The 1,200-ton BABAR particle detector is a similarly complex device that must be carefully adjusted.

"But the entire B Factory, both collider and detector, has come on line smoothly and in record time, to the great credit of the machine builders and the delight of the hundreds of BABAR scientists now trying to cope with the flood of data," says Dorfan.

Thanks to the efforts of a team led by physicist John Seeman, SLAC has now regained several world records for luminosity (the standard measure of collision rates). Data are coming in so fast that one DOE observer likened the experience to "drinking from a fire hose."

But there was a twist to the data analysis: Until the week preceding the Osaka conference, the more than 500 physicists involved in the BABAR collaboration did not even know a key result of their ongoing experiment. Since they were measuring an asymmetry parameter that reflects an anticipated difference between matter and antimatter, the experiment was "blinded" by insertion of a "fudge factor" so that researcher bias would not influence the answer.

The experiment is expected to run through October. "The rapid launch of the B Factory has given us our first glimpse into the new domain of CP violation measurements in the B meson system," says Hitlin. "We hope to double our data by the end of October and to begin to make truly definitive tests of the leading mechanism that has been proposed for this intriguing phenomenon." SR