Nuclear pion photoproduction in theΔresonance region

A measurement of the ${}^{12}\mathrm{C}(\gamma ,{\pi }^{+}{n)}^{11}\mathrm{B}$ reaction in quasifree $\pi$-production kinematic regimes has been performed using tagged photons in conjunction with large solid angle $\pi$ and n detectors. The aim of the experiment was to investigate predicted modifications to the $\Delta$ excitation of nucleons and their subsequent propagation and decay, brought about by the nuclear medium. Differential cross sections are presented for photon energies spanning the $\Delta \mathrm{}\left(1232\right)\mathrm{}$ excitation region. The measurements are consistent with distorted wave impulse approximation calculations in which the amplitude for proton $\Delta$ excitation, followed by $\Delta$ propagation and decay to ${\pi }^{+}+n,$ is reduced compared to that for a free p. However, because of uncertainties in the magnitudes of the final state interactions, it is concluded that improved calculations are required to obtain a quantitative estimate of $\Delta$-medium effects.