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In Search of Antimatter

Two international collaborations have announced definitive answers to one small piece of the universe's matter-antimatter asymmetry problem
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J. W. Stewart
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.

BaBar Detector
Image: SLAC

THE BABAR detector, under construction.

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.

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."

Aerial view of SLAC
Image: SLAC

AERIAL VIEW of the Stanford Linear Accelerator Center.

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."

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