The Belle collaboration, working at the KEKB facility at KEK, has observed a difference in the direct CP asymmetry for decays of charged and neutral B mesons into a kaon and a pion. This result is consistent with previous measurements from Belle and BaBar, but is more precise.
Two types of CP violation have previously been observed in two neutral meson systems, the K0 and B0. In these, the CP violation arises either in the mixing between the K0 or B0 and its antiparticle or – in direct CP violation – in the decay of these neutral mesons. The observed effects in both cases are larger for the B0 system than for the K0 system, but they are consistent with the Standard Model and the mechanism for CP violation first proposed by Makoto Kobayashi and Toshihide Maskawa.
Now the Belle collaboration has found that direct CP violation differs between the charged decay B± →K±π0 and the related neutral decay to K±π-+. In 535 million BB pairs observed at KEKB, Belle found 2241±157 K+π– and 1856±52 K–π+ events, leading to an asymmetry for BB–0→K–π+ versus B0→K+π– of A0 = –0.094±0.018±0.008, which favours B0→K+π–. For the final states expected for the corresponding charged decays the collabroation found 1600+57/–55 K±π0 events, giving an asymmetry A± = +0.07±.03±0.01, with more K–π0 events. The opposite signs of these two asymmetries suggest that different CP violation effects are at work in charged and neutral B mesons.
The causes of this difference in CP asymmetry is uncertain. The large observed deviation might be explained by either strong interaction effects or new physics – specifically a new source of CP violation, something that is needed to explain the domination of matter over antimatter in the universe. To understand whether new physics is indeed involved in B→Kπ decay, further study of CP violation in other modes is needed. Direct CP violation in B0→K0π0 and mixing in the BsBsbar system would be good candidates, but experimental measurements on these systems are not yet precise enough, and much more data are needed. The search for new physics in CP violation will be one of the major goals of the B factory upgrade at KEK as well as other future B physics facilities.