Core-Excitation Effects in Pion Double Charge Exchange

Differential cross sections for the pion double-charge-exchange reaction ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}({\pi }^{+},{\pi }^{-}){}_{}{}^{42}{}_{}{}^{}\mathrm{Ti}(\mathrm{g}.\mathrm{s}.\mathrm{})$ have been measured at ${\theta }_{\mathrm{lab}}=5\mathrm{}°$ at several energies. For $T\left({\pi }^{+}\right)<\mathrm{}200\mathrm{}$ MeV, $\sigma \left(5\mathrm{}°\right)$ are found to be more than an order of magnitude larger than those predicted by conventional double-charge-exchange calculations. It is shown that the observed enhancements can be explained in terms of coherent contributions due to terms proportional to ${\rho }^2\mathrm{}\left(r\right)\mathrm{}$ in the pion-nucleus potential whose importance is dramatically enhanced in the presence of large core-excitation components in ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$(g.s.).