In the scattering of positive pions by nuclei the double-charge-exchange (DCX) reaction creates the possibility of the formation of pionic atom states in the vicinity of the threshold of the reaction. These quasistationary states can manifest themselves as resonances in the elastic scattering cross section. The strength and shape of these resonances are strongly affected by the instability of the nucleus created in the DCX channel. It is shown that this mechanism can explain oscillation structures in the excitation function observed in scattering of low-energy positive pions from ${}^{12}\mathrm{C}$.

• Received 4 May 2013

DOI:https://doi.org/10.1103/PhysRevC.88.054608