Pion double charge exchange onHe4

We have examined the reaction ${}_{}{}^4{}_{}{}^{}\mathrm{He}({\pi }^{\pm }, {\pi }^{\mp })4\left(\left\{p\right\}\left\{n\right\}\right)$ in a model in which there occur two single $\pi -N$ charge-exchange scatterings with intermediate off-shell pion propagation between scatterings. Separable $\pi$-nucleon $t$ matrices having off-shell form factors were used with fully antisymmetrized nuclear wave functions. Spin-flip of the struck nucleons was included. Both angular distributions and total cross sections were calculated and compared with available data.