 Short-lived neutral kaon

The short-lived neutral kaon $K_S$ is one of the two propagating eigenstates of the effective neutral kaon mixing problem. It is a coherent combination $p|K^0⟩+q|{\overline{K}}^0⟩$, with coefficients depending on the phase convention and normalized so $|p{|}^2+|q{|}^2=1$.

In the limit of exact CP symmetry, it is approximately the CP-even combination and readily decays to two pions; the corresponding long-lived state has different allowed decay channels. Small CP violation makes the exact identification with a CP eigenstate imperfect. The observation of two-pion decays in the nominally long-lived beam was therefore evidence for CP violation, not a redefinition of the short-lived state.^[ref]