First observation and precision measurement of direct CP violation: the experiments NA31 and NA48

The CP violating decay of the CP odd eigenstate of the neutral kaon into two pions has first been observed by the NA31 experiment at CERN. The strength of direct CP violation has been measured precisely more recently as expressed by the parameter $|\epsilon′/\epsilon|=(1.7 \pm 0.2) \times 10^{-3}$ . This implies a difference in the di-pionic decay rates of neutral kaons and their antiparticles at a level of $10^{-5}$.