Neutral kaon regeneration ($|K_2⟩⟼\frac{t-\bar{t}}{2}|K_1⟩+\frac{t+\bar{t}}{2}|K_2⟩$)

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 $t|K^0⟩+\bar{t}|{\bar{K}}^0⟩$ 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.^[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.