In 1964, James Christenson, James Cronin, Val Fitch and René Turlay discovered that some of the equations are not CP invariant. It is a small effect, but nevertheless it exists.
The effect was spotted in the decays of a particle called the meson (or the neutral kaon). The consists of a down quark and a strange antiquark (). Its antiparticle, the , consists of a down antiquark and a strange quark (). Now, one does not usually have either a or a . Instead, they exist together in a kind of merged state. A neutral kaon 'dances' back and forth between being a and a . There are two different ways in which it can do this. The difference becomes apparent when the kaon decays.
One version of the dance - the long - decays into 3 pions with the relatively long lifetime of about 10-7 seconds. The other - the short - decays into 2 pions with a relatively short lifetime of about 10-10 seconds:
Although it requires quantum mechanics to explain it fully, all of this was well understood. The surprising discovery made in 1964 was that occasionally a long would decay into 2 pions (about once every 500 decays). This cannot happen if CP invariance is obeyed.
We have already said that the long 'dances' back and forth between being a and a . This is similar to interference in waves. The long is a mixture of a wave and a wave. Now, if the equations were CP invariant, and one were to look at the dance in a mirror and swap round the and , then the dance should look exactly the same. However, this is not the case. CP violation means the dance looks different, and it is this difference that enables the unexpected decay to occur.