The $3\pi$ Mass Spectrum in the Reaction ${\pi }^{-}p\to p{\pi }^{+}{\pi }^{-}{\pi }^{-}$ at 13 and 20 GeV/c.

In the $3\pi$ mass spectrum from the reaction ${\pi }^{-}p\to p{\pi }^{+}{\pi }^{-}{\pi }^{-}$ at 13 and 20 GeV/c, we observe an unstructured $A$ enhancement and the production of the $\pi \mathrm{}\left(1640\right)\mathrm{}$ meson. Our data for the ${\pi }^{-}{\rho }^0$ mass are compared with the predictions of the one-pion exchange, the diffraction-dissociation, and the double-Regge-pole models. Only the double-Regge-pole model is able to reproduce the shape of the $A$ enhancement.