= Neutral-kaon projection interference asymmetry
{title2=$\mathcal R=2\operatorname{Re}r/(1+|r|^2)$}
In the <exchange-positive CP convention for neutral kaon mixing>, a coherent source with <decay amplitudes> $A$ and $B$ for $K^0$ and $\bar K^0$ has projected amplitudes $(A+B)/\sqrt2$ and $(A-B)/\sqrt2$. For equal <phase space>, their <decay-rate asymmetry> is $\mathcal R=2\operatorname{Re}r/(1+|r|^2)$, $r=B/A$. Expanding the squared moduli proves the identity. Knowledge of $|r|$ alone cannot fix the asymmetry: $r=i|r|$ gives zero, whereas $r=|r|$ gives constructive interference in the symmetric channel.
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