Meson-Nucleon Scattering and Nucleon Isobars

The scattering (including charge exchange) of ${\pi }^{-}$ mesons in hydrogen rises from 18 millibarns at 60 Mev to a broad plateau of about 60 millibarns at 200 Mev, and is smaller than the ${\pi }^{+}$ scattering at 60 Mev in the ratio of 0.63±0.09. The general features of the ${\pi }^{-}$ scattering, except for the high energy plateau, are given qualitatively by pseudoscalar theory with pseudovector coupling in the weak coupling limit; the ratio of ${\pi }^{-}$ to ${\pi }^{+}$ scattering predicted by this theory in the weak coupling limit is, however, 1.67, which is much higher than the experimental result. A phenomenological theory of the scattering is developed by using the methods of Wigner and Eisenbud and imposing the restrictions of charge symmetry. By using the qualitative assignment of the resonance levels parameters as given by weak and strong coupling theory, satisfactory agreement with experiment is obtained. It is concluded that the apparently anomalous features of the scattering can be interpreted to be an indication of a resonant meson-nucleon interaction corresponding to a nucleon isobar with spin $\frac{3}{2}$, isotopic spin $\frac{3}{2}$, and with an excitation of 277 Mev.