By Peter Weiss
Physicists have finally answered a decades-old question about the difference between matter and antimatter. Yet their finding only deepens the mystery of why the universe contains so much more matter than antimatter.
Last week, researchers at the Stanford (Calif.) Linear Accelerator Center (SLAC) announced they had proved that hefty subatomic particles, known as B mesons and anti-B mesons, decay into lighter particles in slightly different ways.
This disparity, called the charge-parity (CP) violation, first became apparent in 1964 among particles known as K mesons. After that discovery, some theorists proposed that this type of disparity may solve a cosmic mystery: How is it that the universe is almost solely made of matter even though it theoretically burst into being with equal shares of matter and antimatter?
A pathway to an answer opened in the 1970s when theorists predicted that B mesons also would exhibit CP violation. That prediction has since been incorporated into the main theory of particle physics, known as the standard model. But researchers have lacked the technology for actual tests.
Since 1999, physicists working at giant particle accelerators at SLAC and at the KEK High Energy Accelerator Research Organization in Tsukuba, Japan, have raced to test that prediction (SN: 3/3/01, p. 143: Physicists get B in antimatter studies). While expecting to find CP violation, they were eager to see whether their measurements would disagree with the amount of CP violation predicted by the standard model. If so, physicists might find an explanation for the missing antimatter.
The SLAC team analyzed the aftermath of 32 million B meson-producing collisions between electrons and their antimatter counterparts, dubbed positrons. From those crashes, the experimenters have calculated a value of 0.59 ± 0.14 for a parameter known as sin 2b. Sin 2b would have equaled zero if there were no CP violation among B mesons, and the standard model predicts between 0.50 and 0.85.
Since the SLAC figure overlaps the sin 2b of the standard model, a major piece of the model has been confirmed, says SLAC team member A.J. Stewart Smith of Princeton University. The new result has only 0.003 percent chance of being a statistical fluke, the team reports. The KEK team plans to release what physicists expect to be comparable results next week.
Although the standard model has held up so far, Smith says, physicists intend to investigate other types of B mesons and their behavior for clues to the missing antimatter’s fate.
