Does Particle and Anti-particle Behave Symmetric in B-mesons?



Fumihiko Takasaki


In 1964, it was found that the behavior of particles and their anti-particle partners is not identical in the neutral K-meson decays. This discovery is known as a violation of the CP symmetry in physics terminology. There have been many attempts to understand this phenomena. In the early 1970s, Japanese physicists, M. Kobayashi and T. Maskawa, tried to explain it by introducing six quarks grouped in three sets of quark doublets in early 1970s. They proved that if quark mixing is the origin of CP violation, it is necessary to have at least three sets of quark doublets. It is remarkable that their observation was made when only three quarks were known to exist. The discovery of remaining three quarks supports strongly the Kobayashi-Maskawa hypothesis. However, since attempts to find CP violation in processes other than the two decay modes of the neutral K-mesons have failed, there have been no way to prove or disprove this hypothesis. How to understand CP violation has been a big problem of particle physics. A.I. Sanda and A. Carter pointed out that large CP violation effects can be observed in the B-meson decays based on the hypothesis of Kobayashi and Maskawa. One of their observation is that there should be a difference in the decay probability of neutral B-meson decays and that of neutral anti-B-meson decays. Inspired by this study, people of KEK, the High Energy Accelerator Research Organization, have built an accelerator complex, KEKB, to produce copious pairs of B-meson and anti-B-meson by the collision of electron and positron beams in Tsukuba, Japan. The construction started in 1994 and the experiment, Belle, started in 1999. Since then, more than 18 million B-meson pairs have been observed by the Belle experiment before the end of March 2001. We have analyzed a data sample of 11 million B-meson pairs and observed that CP violation seems to exist in B-meson decays with a confidence level of 95%. This observation supports the Kobayashi-Maskawa hypothesis


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