The Belle experiment(see figure below) has been taking data since June last year, and accumulated data of about 7 million B meson pairs. In these data, we found 98 B mesons decaying into a special decay mode, and after the detailed analysis, we observed an indication that decay of a particle is different from the decay of an antiparticle. This result was reported at the International Conference on High Energy Physics held in Osaka in July. Although the number of the events we have analyzed is not enough yet, more conclusive result will be obtained in near future, because the performance of the accelerator is being improved significantly in this summer.
It is very interesting to study the difference of the nature between particle and antiparticle, not only because it provides us with the deeper understanding of the particle physics, but also because it is relevant to the mystery that there are no antiparticles in the present universe, while the same numbers of particles and antiparticles are believed to exist in the early universe. The "asymmetry" between particle and antiparticle is one of the key ingredients to explain the present universe that consists only of particles.
Such an asymmetry is called "violation of CP invariance" in the particle physics. To our best knowledge, the physics processes keep this CP invariance with only one exception. In 1964, tiny violation of this invariance was discovered in decay of neutral K particle. Makoto Kobayashi(KEK) and Toshihide Masukawa(Kyoto Univ.) proposed a theory to explain this CP violation by introducing six types of quarks and the mixing among them. According to this theory, a large CP violation is expected in the decays of B mesons, and therefore, it is essential to measure the CP violation in B mesons to test the validity of the theory. This is the main goal of the Belle experiment.
Figure Belle detector | | | | Click for the large image(91KB) | |
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