Parity selection rule for a two-body decay
= Parity selection rule for a two-body decay
If parity is conserved when a particle of intrinsic parity $\eta_i$ decays into two particles of intrinsic parities $\eta_1,\eta_2$, then their relative partial wave obeys
$$
\eta_i=\eta_1\eta_2(-1)^\ell.
$$