Colorful collision. Particles collide in a BaBar experiment.

Darn! Matter and Antimatter Behave as Predicted

The prevailing theory of fundamental forces works really well, but it can't explain how the early universe produced so much more matter than antimatter. So physicists have been hoping that they might find a flaw in their theory that could provide a clue to the mystery of the matter majority. Yet the latest experiment shows that matter and antimatter behave exactly as the so-called Standard Model predicts--making the experiment a bittersweet success.

According to the Standard Model, a particle of antimatter and its matter cousin share a near, but not quite perfect symmetry. In 1964 physicists found that particles called K mesons decayed in ways not precisely mimicked in the decays of anti-K mesons. In the 1970s and ‘80s, theorists figured out how to include an explanation of this "CP violation" as they pieced together the Standard Model. But only in the past few years have physicist been able to test that explanation by studying another particle-antiparticle pair that should behave in similar ways. In 1999, researchers began studying particles called B mesons and their antimatter partners with particle colliders at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, California, and at the KEK laboratory in Tsukuba, Japan.

Here's what they were looking for. Within the mathematics of the Standard Model, various angles quantify how far matter and antimatter are out of kilter. Physicists working with SLAC's BaBar particle detector measured one key angle, b, that reveals itself when B and anti-B mesons decay into certain combinations of particles. They have found that b is about 24 degrees, just as the Standard Model predicts, they reported today at the International Conference on High Energy Physics in Amsterdam. Researchers working with the Belle particle detector at KEK will present their value for b at the conference on Monday.

Scientists usually cheer when the results of their experiments closely match their theories. But in this case many physicists were quietly hoping the data would defy the tried-and-true Standard Model and give them a tantalizing surprise, says Peter LePage, a theoretician at Cornell University in Ithaca, New York. "The Standard Model is just frustratingly successful," LePage says. Michael Peskin, a theorist at SLAC, says that other, more precise measurement with B mesons might still provide subtle hints about how matter overwhelmed antimatter in the early universe. "We're not at the end of the story yet," Peskin says.

Related sites
The BaBar Web page
The Belle web page
ICHEP02 meeting Web page
The Particle Adventure