Pionic Double Charge Exchange onN=ZDoubly Closed Shell Nuclei

Abstract

The $({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}})$ reaction on the isoscalar, doubly magic nuclei ${}^{16}\mathrm{O}$ and ${}^{40}\mathrm{Ca}$ has been measured in the energy range ${T}_{\ensuremath{\pi}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}45--90\mathrm{MeV}$. 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 $\ensuremath{\pi}\mathrm{NN}$ resonance in the course of the double charge exchange process. The deduced spreading widths are in accordance with estimates of the ${d}^{\ensuremath{'}}$ 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 ${E}_{x}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.1(2)\mathrm{MeV}$ 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 ${E}_{x}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.03\mathrm{MeV}$.