2/13/01

Michael Riordan, SLAC (650) 926-3990;
michael@slac.stanford.edu

Physicists publish first results from the B Factory at Stanford

The international BABAR Collaboration at the Stanford Linear Accelerator Center (SLAC) has just released initial results on the behavior of subatomic particles known as B mesons. Based on nearly 25 million collision "events" accumulated in 1999 and 2000 at this Department of Energy laboratory, these are the most precise results obtained to date on the short-lived particles, which are expected to provide important clues about why our Universe contains far more matter than antimatter.

"This is the culmination of more than a decade of efforts by hundreds of scientists and engineers around the world," said SLAC Director Jonathan Dorfan. "Our experiment is beginning to yield tantalizing physics results."

Designed and built by a large team of nearly 550 physicists from nine countries, BABAR is a 1,200-ton particle detector that measures subtle distinctions between the decays of B mesons and those of their antimatter counterparts, or anti-B mesons. If such asymmetries occur, they should provide additional evidence for a mysterious phenomenon known as "CP violation," which has intrigued physicists for decades.

This intrinsic difference between matter and antimatter was first discovered in a 1964 experiment at Brookhaven National Laboratory. Three years later, Russian physicist Andrei Sakharov used CP violation to explain how our present, matter-dominated Universe could have emerged out of one that contained equal amounts of matter and antimatter during the earliest moments of the Big Bang.

Caltech physicist David Hitlin presented the new results today in a SLAC seminar. In a paper just sent out for publication in Physical Review Letters, BABAR physicists provided their best value of an asymmetry parameter known as sin 2b ("sine-two-beta"), which they determined by comparing how rapidly B and anti-B mesons decay into a set of specific final states. Their result is sin 2b = 0.34 ± 0.20, which is about twice as accurate as previously published values.

"This is the most challenging answer that Nature could have given us," remarked Princeton University physicist Stewart Smith, spokesman for the BABAR Collaboration. While it favors a matter-antimatter asymmetry, the result is not yet completely convincing. More data on B and anti-B meson decays will be needed to reach a definite conclusion, one way or the other.

The precision of the BABAR result was made possible by the better-than-expected performance of the PEP-II B Factory at SLAC. Built in collaboration with Lawrence Berkeley and Lawrence Livermore National Laboratories, this machine collides beams of electrons and positrons of unequal energies. During its first 18 months of operation, the B Factory quickly reached its projected luminosity -- a measure of the collision rate between electrons and positrons.

"The B Factory is achieving its promised performance, and it will exceed that promise during the coming year," said Dorfan. The collider has just begun its second experimental run, expected to continue through August.

"The B Factory is operating like a fire hose, flooding us with data," said Smith. "BABAR physicists are delighted by the opportunities to do great physics that we have been given so early in the game, and we are working extra hard to cope with all the challenges that this embarrassment of riches presents."

By accumulating many more millions of events, the BABAR physicists will refine their measurements of sin 2b and other key parameters, including another asymmetry germane to studies of CP violation. They also will search for rare events in which B mesons decay into exotic final states that so far have not been observed in other experiments.

The BABAR Collaboration includes physicists from 73 institutions in Canada, China, France, Germany, Great Britain, Italy, Norway, Russia and the United States. "The foreign contributions, both financial and scientific, have been absolutely crucial to the success of this experiment," noted Smith.

SLAC 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 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.

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By Michael Riordan