CPT symmetry

After it was shown that parity was not conserved in the waek interaction, parity was combined with "charge conjugation" which told whether you were dealing with a particle or an antiparticle. It was shown that charge and parity (CP) together was conserved. But then it was found that Neutral kaons did not conserve CP, but did if you added time (T). Thus CPT.
 Val Logsdon Fitch, James Watson Cronin United States 1964 CPT symmetry

After it was shown that parity was not conserved in the waek interaction, parity was combined with "charge conjugation" which told whether you were dealing with a particle or an antiparticle. It was shown that charge and parity (CP) together was conserved. But then it was found that Neutral kaons did not conserve CP, but did if you added time (T). Thus CPT.