Modification of the One-Pion-Exchange Model forπ−+p→π−+π++n

The one-pion-exchange model for the production process ${\pi }^{-}+p\to {\pi }^{-}+{\pi }^{+}+n$ is modified to include a tensor component for the exchanged pion off its mass shell. This modification is suggested by the hypothesis of the Regge pole description for the pion. Experimental evidence for the deviation from the ordinary one-pion-exchange model can be understood in the present theory provided the final-state ${\pi }^{-}{\pi }^{+}$ pair contains an appropriate mixture of $J=0\mathrm{}$ and $J=1\mathrm{}$ ($\rho$ meson) amplitudes.