 Solution

The charged pion has valence content ${\pi }^{-}=d\bar{u}$. Its constituents annihilate into a virtual W boson, followed by the lepton pair:

The quark vertex carries the $u$--$d$ Cabibbo-Kobayashi-Maskawa matrix element, and the leptonic vertex is the weak charged current. The pion's nonperturbative spatial structure is encoded in the pion decay constant when evaluating the decay amplitude.