Most people probably take it for granted that the universe is made mostly of matter and not its opposite, antimatter. But not particle physicists. For decades, members of this elite group have been grappling with the question of why matter dominates our universe. If the big bang and the universe it created were symmetrical, they reason, equal amounts of matter and antimatter should exist. But it just didn't work out that way.
Those of us who are not particle physicists should be grateful for the imbalance. If equal amounts of matter and antimatter in fact existed, they would promptly combine and annihilate each other in a burst of energy. Indeed, a particle and its antiparticle are equal yet opposite in every way and should behave similarly. Physicists call this property symmetry. The fundamental symmetries of nature include charge and parity, or handedness. So an electron should behave the same way as a positron (its antiparticle), just as a particle in a right-handed coordinate system should do the same things if viewed in a three-dimensional mirror, thereby putting it in a left-handed system. Because our matter-dominated universe exists, though, physicists have been looking for situations in which particles and their antiparticles break from symmetry.
Now two international collaborations have each announced definitive answers to one small piece of this matter-antimatter asymmetry problem. After close to two years of data collection, the BaBar collaboration, based at the Stanford Linear Accelerator Center (SLAC), and the BELLE Collaboration, based at the KEK laboratory in Tsukuba, Japan, both measured one type of asymmetry in particles known as B mesons. The BaBar group revealed their result first, on July 5, at a physics meeting in Colorado. Less than three weeks later, the BELLE group unveiled their findings at a meeting in Rome. Both groups have since submitted papers to the journal Physical Review Letters to be published August 27. The two groups started accumulating data less than a week apart back in the spring of 1999. Ever since, they¿ve been "very strong, equal and friendly competitors" in the race to find asymmetry in B mesons, according to physicist Stewart Smith, a spokesman for BaBar.
Until 50 years ago, the laws of physics assumed that both charge symmetry and parity symmetry were conserved independently. In 1957, when Madame Chien-Shiung Wu of Columbia University discovered a situation in which parity was violated, the physics community adjusted by assuming that, together, charge and parity (CP) must be conserved. It was a perilous adjustment. In 1964 Val Fitch and James Cronin discovered CP violations in particles called neutral K mesons, and physicists once again scurried to explain the findings. Makoto Kobayashi and Toshihide Maskawa soon developed a theory that explained the CP violation in K mesons and has since been accepted as part of the Standard Model, the complex set of equations that physicists use to describe our universe.
Still, seeing the phenomenon in only one particle for 37 years left physicists with an uneasy feeling. "There is something a little bit uncomfortable about only having this effect in one particular particle," says Ed Blucher, a physics professor at the University of Chicago who studies CP violation in K mesons. "There is this sort of nagging, ¿Is it just something peculiar about this one system?¿ So, in a very basic way, seeing that this effect occurs in another particle system is very interesting." The theory that incorporates CP violation in K mesons¿termed the Kobayashi-Maskawa matrix¿predicted that CP violation should also happen in the B meson system. Problem was, no one had been able to find it. Determining whether CP violation occurs in B mesons, which are 10 times heavier than their K counterparts, is exactly why scientists initiated the BaBar and BELLE collaborations.