The two accelerators, unlike previous attempts to measure CP violation in the B system, are what are known as asymmetric B factories. In them, an electron beam and a positron beam of different energies crash into one another to manufacture B meson pairs. At SLAC, the electron beam is maintained at nine billion electron volts, while the positron beam scoots around the two-kilometer track at a mere three billion electron volts. The resulting B meson pairs¿like the aftermath of a demolition derby crash¿stick together and move with a finite, intermediate speed. In the case of the B meson pairs, that speed is nearly half the speed of light. "If something¿s moving with half the speed of light, even if it only lasts a few trillionths of a second, it goes a quarter of a millimeter, in our case, on the average," says Smith, a Princeton University physicist. "And by doing that, we can measure the times at which the B and the anti-B particles decay."
The difference in the decay rates is a measure of the asymmetry between matter and antimatter. If both particles decay to the same final state in the same amount of time and in the same way, no asymmetry exists and CP is conserved. But with their new results, the BaBar and BELLE groups definitely proved that Bs and their anti-B counterparts (termed B bars, the inspiration for the collaboration's name) decay differently. Although their numbers are slightly different¿BELLE¿s is bigger¿they are still relatively consistent both with each other and the prevailing ideas. "We and SLAC both basically proved the CP symmetry is definitely broken in the B system," says Hiroaki Aihara, a physics professor at the University of Tokyo who has worked with BELLE since its inception. "The actual numbers, why we are different from them, we don¿t really know yet, and where this difference comes from remains to be seen. Once we get more data, we can both find out. Things may change in the future."
By looking at one particular final state of the decay of Bs and B-bars, called J/psi K-short, the collaborations probed the differences between the two. The researchers chose this particular decay mode for two reasons: First, the ensuing calculations are straightforward, whereas other measurements are, in the words of Blucher, "a nightmare" to interpret. Second, it is relatively easy to measure. But even though this decay mode is the easiest to find, it occurs only approximately once in every 100,000 decays. Both groups manufactured more than 31 million pairs of B mesons in order to find enough events to be statistically sure that what they were seeing was actually CP violation.
The amount of asymmetry measured in the B system is not enough¿by many orders of magnitude¿to explain the cosmological problem in the universe. The amounts determined by both groups are consistent, however, with the model invented in the 1970s that explained the CP violation discovered in the Kaon system and predicted it for the B system. "The level [of asymmetry] is consistent with the Standard Model," Smith says. "But is not enough to say there is nothing beyond the Standard Model."
In fact, even though the two collaborations proved CP violation occurs in the B system, they are far from finished. The general plan over the next 10 to 15 years, physicists say, will be to try and make the measurements that have already been done more precise and also to measure a variety of other parameters in order to further test the current models. BaBar¿s Smith agrees that the current results are a beginning point, not an end one. "I think it¿s just really starting because many big changes in our outlook in science started with small discrepancies," he says. "Clearly, we were hoping to find some huge discrepancy right off the bat, but nature¿s more subtle than that."