In nuclear physics lessons I've been taught that pions are pseudoscalar particles and thus their intrinsic parity is odd. The professor said that an experimental proof of this can be derived observing the process: $$ \pi^- + D \quad \longrightarrow \quad n + n $$
where the pion is considered as an s-wave ($\ell = 0$ ) and the deuteron is in triplet state ($s=1$).
I can't understand why this decay can prove pions have a negative intrinsic parity.