Physicists, through experience, have developed a preference for the theories of nature that possess a high degree of symmetry. The study of beta-decay (on the other hand) revealed a very surprising feature of the weak interaction in that this process does not obey the principle of spatial reflection symmetry. Otherwise known as parity conservation, this symmetry principle maintains that fundamental processes should be the same under a spatial inversion of all vector parameters. In such a transformation, position vectors (for example) are reflected through the origin and motion vectors such as momentum are reversed. Consider a simple example of a ping-pong ball that scatters elastically from a bowling ball positioned at the origin. If initially the ping pong ball is incident from the left, under parity inversion of the experiment it would approach the bowling ball from the right. One would not expect the results of the experiment to change under such an inversion of circumstances, but in fact particles that interact through the weak force exhibit just this sort of odd behavior when they interact or decay. This discovery shocked the physics community in the late 1950's. Despite the astounding progress that has been made in the understanding of the fundamental forces over the past fifty years, the origin of parity violation in the weak interaction is still not understood and remains one of the mysteries of modern science.
The first demonstration of parity violation came from Madame Wu et al. They showed that in the beta-decay of 60Co the emitted electrons tend to come out anti-aligned with the nuclear spin. Under a parity inversion of this experiment, the electrons would come out in the opposite direction. Therefore, the process that causes beta-decay must violate parity conservation symmetry.