CP Violation Group at the University of Chicago

Basic theory

Kaons are mesons formed by a strange (or anti-strange) quark and an up or down quark. They have strangeness of ± 1. Both charged kaons (K⁺, K^-) and neutral kaons (K₀, K ₀) exist. These neutral kaons are distinguished by their strangeness: S(K₀)=1, S(K₀)=-1. The neutral kaons offer a very interesting subject for studying. During particle - antiparticle oscillation, K₀ -> K₀, and K₀ -> K₀.

Therefore, these are not CP eigenstates.

Theoretically, in order to get them, it should be enough to use their linear combinations. (We use Dirac notation to indicate the wave fuction.)

K₁ primarily decays to 2 π which occurs quickly. According to the CP eigenvalue, K₂ primarily decays to 3 &pi which occurs on a slower timescale. In 1964, Cronin and Fitch observed that CP is not conserved in weak interaction in their experiment of a decay of a long lived neutral kaon to 2 π .

This means that the kaons that are CP eigenstates are linear combinations of these two observed in experiments (such as our, for instance) are a mixture of K₂ with a small amount of K₁, which we call K_L.

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