= Neutral-pseudoscalar vector-pion charge-conjugation selection
A neutral self-conjugate pseudoscalar with $C=+1$ cannot decay through charge-conjugation-conserving interactions into a neutral $C=-1$ vector and a neutral $C=+1$ <pion>. The two final particles are distinct and each maps to itself under <charge conjugation>, so their total C eigenvalue is the product, without an exchange factor $(-1)^L$. Their relative orbital <angular momentum> still enters their total <parity>. For $0^-\to0^-1^-$, $L=1$ gives a CP-violating transition if a weak amplitude is retained.
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