B-Decay Experiments Show Clear Violation of CP Symmetry

In March, back-to-back papers in Physical Review Letters reported the measurement of CP symmetry violation in the decay of neutral B mesons by groups in Japan and California. ^1,2 (See Physics Today, May 2001, page 17 .) Now the word “measurement” has been replaced by “observation” in the titles of two new back-to-back reports ^3,4 by these same groups in the 27 August Physical Review Letters. That is to say, with a lot more data and improved event reconstruction, the BaBar collaboration at SLAC and the Belle collaboration at KEK in Japan have at last produced the first compelling evidence of CP violation in any system other than the neutral K mesons.

The evidence for CP violation in the BaBar and Belle experiments is summarized by a nonvanishing value for the measured parameter sin 2β. In March, each collaboration reported that its measured sin 2β was about 1.7 standard deviations above zero. That was encouraging, but not yet convincing.

Now that each experiment has created more than 30 million $\overline{B}$ pairs in its asymmetric electron-positron collider, the evidence for CP violation in B decays is quite compelling. BaBar reports a sin 2β of 0.59 ± 0.15, four standard deviations from zero. With essentially the same quoted uncertainty, Belle reports an even larger CP violation, namely sin 2β = 0.99.

If one combines the two results without worrying about the marginally uncomfortable discrepancy between them, one gets a world average of 0.79 ± 0.1. That’s in good agreement with the sin 2β = 0.7 ± 0.2 predicted without much precision by the standard model of particle theory. As the experimental errors continue to shrink with the accumulation of data over the next few years, so will the uncertainty of the theoretical prediction, which actually depends on ongoing worldwide measurements of related but less exotic processes.

So now, 37 years after the unanticipated discovery of a small amplitude for CP violation in neutral-kaon decay, we have at last a second arena in which to study this important phenomenon. But the very attribute that suits neutral B mesons so well to this study—namely, the almost maximal value of sin 2β—is not an unmixed blessing. A phenomenon so generously favored by the standard model makes it difficult to glimpse any small nonstandard mechanism that might guide us toward a more encompassing theory. Therefore Belle and BaBar are also seeking to measure smaller CP -violating parameters related to rare B decay modes, in an effort to find small departures from standard-model predictions.