The Discovery of Direct CP Violation

Soon after the discovery in 1964 of the non-conservation of CP symmetry in the neutral kaon system, the hunt was launched for a component arising from direct violation on top of the dominant effect due to mixing. It took almost 20 years until the first evidence of a signal was reported by NA31 and another 10 years to establish the effect with a significance of more than 5 standard deviations. This article describes the beams, detectors and analysis methods used by the two CERN experiments, NA31 and NA48, which made key contributions to these results and established new standards for precision measurements.