On 5 August 1964, physicists from around the world gathered in Dubna for the start of the 12th International Conference on High-Energy Physics (ICHEP). It was to be an historic meeting, with a special session at which James Cronin presented some remarkable results, published not many days earlier, from an experiment on neutral kaons at Brookhaven Laboratory. Specifically, the experiment had shown that the long-lived neutral kaon can decay into two pions, implying the violation of CP symmetry – an subtle difference between particles and antiparticles.

The discovery raised many questions about the origins and size of the unexpected effect. More refined experiments followed, giving more precise measurements for the different decay modes. By the time of the 17th ICHEP in London in 1974 all results agreed perfectly with the predictions of the so-called superweak model. However, a new theory that accounted for CP violation was already in the air – and with it new challenges for a new generation of experiments.

In 1973 Makoto Kobayashi and Toshihide Maskawa produced the first theory of CP violation in the context of the Standard Model. With a third generation of quarks, it provided for the phenomenon both in the mixing of neutral kaons (indirect CP violation) and in their decays (direct CP violation). This prompted renewed efforts to search for direct CP violation, which was previously unseen and was not required by the superweak model.

At CERN, the NA31 experiment found the first evidence in 1988. Ten years later, the experiment’s successor, NA48, clearly established direct CP violation in the neutral kaon system, so helping to pin down the phenomenon in the context of the Standard Model of particle physics.

For more about CP violation’s early days and the NA31 and NA48 experiments see this issue of CERN Courier.