At the time of the Big Bang, 13.7 billion years ago, heavy matter and antimatter particles, whose nature still remains unexplored to date, were created in equal numbers from a tremendous energy density. All these heavy particles decayed within a very short period of time into well-known particles of matter, namely quarks, leptons, and their antiparticles. Particles and antiparticles annihilated each other converting their masses into light (photons). According to this theory, today's matter-dominated Universe, with its atoms, molecules, stars and galaxies, should not exist at all. Scientists explain this “victory” of matter over antimatter, to which we owe our existence, with a tiny imbalance (asymmetry) in the decay between the heavy particles and their antiparticles: In their decays, the heavy particles ultimately produced more protons than the heavy antiparticles produced anti-protons. The causes of this imbalance will be investigated with the new Belle II experiment at the SuperKEKB accelerator in Japan.
The new SuperKEKB accelerator and the large detector, Belle II, constitute a milestone in the investigation of the matter excess (CP violation) in the Universe. In SuperKEKB, bunches of particles of matter (electrons) and their antiparticles (positrons), with energies up to 8 GeV, are brought to collision at rates which are 40 times larger than in the previous KEKB accelerator. The particles being created in the collisions and the decay products formed are measured and analyzed in the Belle II experiment. With the high statistics provided by SuperKEKB, scientists are hoping to finally find deviations from the predictions of the Standard Model. B mesons, composed of a “heavy” b quark and a “light” anti-quark play a special role here. In SuperKEKB the energies of the electrons and positrons are chosen such that exactly one B meson and one anti-B meson are produced per collision. Due to the high density of the colliding bunches, made possible by the extremely small beam cross sections of SuperKEKB, the B meson pairs are produced in unprecedented large numbers. For this reason, the SuperKEKB accel- erator is also called a “Super B factory”.
CP violation is the subject of various experimental investigations. With the forerunner experiments Belle at the KEKB accelerator and BaBar at SLAC (California), scientists were able to describe the fundamental mechanisms for the decay of B mesons and their anti- particles within the framework of the Standard Model. For the development of the basic theoretical principles of CP violation in the Standard Model M. Kobayashi and T. Maskawa were awarded the Nobel Prize for Physics in 2008. Never-the-less, the Standard Model cannot deliver a satisfactory explanation for the excess of matter observed in the Universe. In the future, the Belle II detector will supply scientists with plenty of much more precise data in order to search for new sources for CP violation beyond the Standard Model.