From the discussion of beta decay it might seem that the joint transformation of parity and charge conjugation, PC for short, should be a symmetry for all systems. We know that its a true symmetry for the nuclear force and for electromagnetic forces. However, in 1963 J. Christenson, J. Cronin, V. Fitch, and R. Turlay discovered that CP is violated in the decay of the neutral kaon.
 

In the experiment of Cronin et al. a beam of nearly pure neutral kaons at 1 GeV/c momentum was injected into a 15 meter long vacuum tube. The short component has a mean travel distance of only a few centimeters. The long component has a travel distance of about 15 m. From the nature of the pion states it was expected that no decays of K-long into 2 pions would be seen if CP were a good symmetry of the weak interaction.

In order that CP be a conserved in neutral kaon decay it can be shown that the K-long component should be a 50% admixture of the neutral kaon plus the anti neutral kaon.

If e = 0 then CP would be conserved, but experiment shows that e is small but definitely non-zero.
 

This violation of CP conservation in the weak decay of the kaon means that time reversal invariance must also be violated in order that CPT be conserved. Hence on the microscopic scale there exist subatomic systems for which time has a unique direction of flow!

There may also be an answer here as to why the universe is dominated by matter. If CP were always conserved for every type of interaction then there should have been equal amounts of matter and anti matter in the early universe. We would then have no matter left in the present universe. This can be iluustrated by the following scenario:

1) neutral kaons and anti neutral kaons are created by the strong interaction.
2) these kaons are initially composed of 50% K-short and 50% K-long kaons
3) The K-short component decays away leaving the K-long component
4) the K-long component can be viewed as a nearly equal mixture of kaons and anti kaons as above except for the slight imbalance of K⁰  .
The neutral kaon and anti neutral kaon decay according to different modes. But no matter which kaon one started with the end result will be an imbalance of positrons created. One could imagine scenarios where the negative pion is captured by a proton to produce a neutron. The neutron is captured by a second proton and forms a stable nucleus( deuteron). This branch contaning pion capture would remove the pion as a source of electrons to counterbalance the excess positron production in the decay of the kaon.
 
 

The existence of CP violation in kaon decay may be part of the explanation why the universe has an imbalance between matter and antimatter.

	Quark structure of hadrons In the decay of the neutral kaon positrons are produced as a decay product. In the quark picture of kaon decay an anti strange quark, emits a W+ which decays into a positron and a  neutrino.
	In the decay of the anti neutral kaon electrons are produced. In the quark picture of kaon decay a strange quark, s, emits a W- which decays into an electron and an anti neutrino.