The motivation behind the Belle experiment
The Universe, as we know it, is comprised of elementary particles such as electron, protons and neutrons. These are the basic building blocks of matter. It is, however, unclear why their respective antiparticles (positrons, antiprotons and antineutrons) are not observed in nature. When a particle and antiparticle are exposed to each other they will annihilate and release energy in the form of radiation.
If the laws of physics were the same for both matter and antimatter we would expect the Big Bang to have produced equal amounts of each. Each particle would then have annihilated with its antimatter counterpart, and the Universe would be composed entirely of radiation. Since we are surrounded by, and made up of, antimatter this is not the case. From this we can assume that the laws of physics treat matter and antimatter differently.
The distinguishing features between a particle and an antiparticle consists of two properties of a particle. The obvious is the charge, in the case of a negative electron and a positive antielectron, or positron, but there is also a property called the parity. Swapping the parity of a system creates a mirror image of a system inverting spatial properties such as the spin of a particle.
As matter an antimatter behave differently, and matter and antimatter differ by their charge (C) and parity (P), we can assume that the laws of physics are not symmetric for transformations of CP for a system. This is a violation of the CP symmetries and is what physicists refer to as CP-violation.
CP violation was discovered in 1964, when it was observed that while a neutral kaon can transform in to its antiparticle and vice versa, the transformations do not occur with the same probabilities.