The NA31 experiment, using a proton beam from the Super Proton Synchrotron (SPS), publishes the first precise results on what is known as ‘direct’ charge-parity (CP) symmetry breaking. This discovery contributes to explaining the tiny difference in the behaviour of matter and antimatter particles. The measurement is later further refined by the NA48 experiment, running at the SPS between 1997 and 2001, as well as by the KTeV experiment at Fermilab in the United States.