Multiparticle parity (${\eta }_1{\eta }_2(-1{)}^{\ell }$)

The parity of a two-particle centre-of-mass state with intrinsic parities ${\eta }_1,{\eta }_2$ and relative orbital angular momentum $\ell$ is

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