Pionic Double Charge Exchange onN=ZDoubly Closed Shell Nuclei

The $({\pi }^{+},{\pi }^{-})$ reaction on the isoscalar, doubly magic nuclei ${}^{16}\mathrm{O}$ and ${}^{40}\mathrm{Ca}$ has been measured in the energy range $T_{\pi }=45\mathrm{}–90\mathrm{MeV}$. The forward angle cross sections of the ground state transitions (GST) show a very pronounced resonancelike energy dependence. The observed energy and angle dependences are well described by the recent hypothesis of the formation of a $\pi \mathrm{NN}$ resonance in the course of the double charge exchange process. The deduced spreading widths are in accordance with estimates of the $d^{\prime }$ collision damping in the nuclear medium. In the reaction on ${}^{16}\mathrm{O}$ in addition to the GST the transition to a $0^{+}$ state at $E_x=2.1\mathrm{}\left(2\right)\mathrm{MeV}$ in ${}^{16}\mathrm{Ne}$ has been observed, which is interpreted as the quadruple isobaric analog state (QIAS) of the $0_2^{+}$ state in ${}^{16}\mathrm{C}$ at $E_x=3.03\mathrm{}\mathrm{MeV}$.