= Two-pion charge-conjugation parity
{title2=$C(\pi^+\pi^-)=(-1)^L$}
For a neutral <pion> pair in a state of definite relative <orbital angular momentum> $L$, <charge conjugation> interchanges $\pi^+$ and $\pi^-$. Relabelling the particle slots reverses the relative momentum and contributes $(-1)^L$. The individual charged-pion phase conventions cancel in the pair. Thus a neutral vector <rho meson> decaying strongly to a pion pair has $L=1$ and $C=-1$. A charged <rho meson> is taken to the state of opposite <electric charge>, so it is not itself a charge-conjugation eigenstate.
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