= Short-lived neutral kaon
{title2=$K_S$}
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 $p|K^0\rangle+q|\overline K^0\rangle$, 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.<Evidence for the 2π Decay of the K₂⁰ Meson by James H. Christenson and others>[1964 long-lived-beam two-pion decay experiment]{ref}
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