Role of second-order optical potential in pion-oxygen scattering

We analyze $\pi$-${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ elastic scattering at pion energies between 40 and 340 MeV. An excellent fit to the data follows from the inclusion of a second-order optical potential. The parameters specifying the second-order potential are found to exhibit marked resonance behavior. The magnitude and the energy dependence of the parameters of the second-order potential obtained for pion-oxygen scattering are, in the main, very similar to that determined for pion-carbon scattering. This result indicates that a universal parametrization of the second-order pion-nucleus optical potential may be possible.