= Neutral kaon regeneration
{title2=$|K_2\rangle\longmapsto\frac{t-\bar t}{2}|K_1\rangle+\frac{t+\bar t}{2}|K_2\rangle$}
<Neutral kaon regeneration> is the production of a short-lived component when a neutral <kaon> beam passes through matter. In the phase convention of <kaon CP eigenstates>, let the forward <transmission amplitudes> of $K^0$ and $\bar K^0$ be $t$ and $\bar t$. Expanding $t|K^0\rangle+\bar t|\bar K^0\rangle$ in the <kaon CP eigenstates> gives the displayed equation. Different forward <scattering amplitudes>, including absorption, can therefore generate a two-<pion> signal even with <CP symmetry>. Ordinary matter is not a <CP>-symmetric preparation.
Tungsten regeneration calibrated the signal; scanning its position sampled acceptance. Helium regeneration was insufficient, bag walls lay outside acceptance, vertex distributions excluded collimator regeneration, and hydrogen data cross-checked the effect.<Evidence for the 2π Decay of the K₂⁰ Meson by James H. Christenson and others>[p.138, tungsten scan; p.140, helium, walls, vertices and hydrogen controls]{ref} These controls are central: the question is whether the long-lived component itself reaches two <pions>, rather than whether material has prepared a fresh short-lived component.
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