The idea of antimatter was first suggested by Paul Dirac in 1930. Matter and antimatter have the same properties but opposite charges; electrons and positrons have the same amount of charge, mass, and spin, but opposite charge. Thus, when matter and antimatter collide, they annihilate each other and only energy remains. During the Big Bang, equal amounts of matter and antimatter should have been created. If all matter and antimatter annihilate, our universe should have been empty. However, astronomical observations suggest that our universe is mostly made of matter while antimatter is scarce. This implies that while antimatter disappeared, matter remained, which is the reason for the existence of galaxies and humans in the universe. This is a puzzling outcome of the universe.

This mystery can be explained only if matter and antimatter have slightly different properties, which is called matter-antimatter asymmetry. The first evidence of matter–antimatter asymmetry was observed in kaon decays in 1964. Kobayashi and Maskawa proposed a theory that explained the origin of matter–antimatter asymmetry of kaon decays and also predicted larger matter-antimatter asymmetries in B-meson decays. The Belle experiment verified the phenomenon predicted by the Kobayashi-Maskawa theory at the turn of the millennium. However, the asymmetries from the Kobayashi-Maskawa theory were insufficient in explaining the amount of the remaining matter in our universe. The Belle-II experiment was designed to explore new mechanisms of matter-antimatter asymmetry that would resolve this mystery.

One approach to probe new mechanisms of matter–antimatter asymmetry is to study decays of b quarks or tau leptons where their magnitudes may be strongly affected by new particles. In particular, some decays of those particles may be greatly suppressed in the present standard theory but these decays may be enhanced by new particles. In such cases, matter-antimatter asymmetry could be caused by new and unknown particles.

Suppressed decays take place very rarely (much lesser than one million). Many particle collisions would be required to study this phenomenon. An extremely powerful accelerator (40 times more powerful than the predecessor), SuperKEKB, is built for this purpose. The Belle experiment is upgraded to the Belle-II experiment to withstand the harsh environment of a powerful accelerator and to collect more data. Such huge data enable us to explore other interesting and rare phenomena.

This manuscript describes analysis methods and their sensitivities for hundreds of decay modes. Some analysis methods are significantly improved by the advantages of modern analysis techniques. This experiment has started recently and will continue to collect data for years to come and the luminosity of the accelerator has already exceeded the previously recorded value. We look forward to exciting discoveries brought by this new endeavor.