Inelastic pion excitation of the $0^{+}$ (7.65-MeV) level in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ is studied using a Kisslinger optical potential in both a distorted-wave impulse approximation and a coupled-channels formalism. In clusion of coupling through the first excited $2^{+}$ state in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ causes a dramatic suppression of this cross section for pion energies below 75 MeV. This explains the unexpectedly small $0_1^{+}$ cross sections recently observed, and may permit detailed testing of pion-nucleus reaction theories.

• Received 28 June 1978

DOI:https://doi.org/10.1103/PhysRevLett.41.1101