In the kaon system, the states with definite CP are the and the , with the the CP-even state and the CP-odd state. The major decay modes in the kaon system are dominated by decays into two and three pions, states with CP=+1 and CP=-1, respectively. Since the phase space for a kaon to decay into two pions is greater than that for the decay into three pions, the lifetime of the was predicted to be much longer than that of the . This state long-lived state was discovered in 1956. CP violation would occur if the long-lived kaon were observed to decay into two pions, the CP-even state.
In 1964 the study of CP violation was born with the discovery of the decay mode of the in the famous experiment of Christenson, Cronin, Fitch and Turlay. The impact of their observation on elementary particle physics has been profound. Here, for the first time, was a mechanism that could distinguish matter from anti-matter. The decay of the long-lived state into two pions occurs at the 0.2% level.