Two-pion charge-conjugation parity ($C({\pi }^{+}{\pi }^{-})=(-1{)}^L$)

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.