ψ′→ψππdecay as a test of partial conservation of axial-vector current

The shape of the ${\pi }^{+}{\pi }^{-}$ mass distribution in the recently analyzed ${\psi }^{\prime }\to \psi \pi \pi$ decay is explained. Although the explanation assumes the known general features of the $\pi \pi$ $s$-wave amplitude, detailed alternatives are not distinguished; rather, it is the on-shell appearance of the Adler zero which is crucial. The extension of this description to other dipion production processes is illustrated with a discussion of low energy ${\pi }^{-}p\to {\pi }^{+}{\pi }^{-}n$.