Energy and mass dependence of low-energy pion single charge exchange to the isobaric analog state.

Abstract

Forward-angle differential cross sections at low energies (\ensuremath{\le}80 MeV) are reported for pion single charge exchange to the isobaric analog state for nuclei from $^{7}\mathrm{Li}$ to $^{120}\mathrm{Sn}$. A minimum in the 0\ifmmode^\circ\else\textdegree\fi{} excitation function near 50 MeV persists for all of the measured nuclei and reflects the analogous minimum in the ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}n$ cross section that is caused by a nearly perfect cancellation of the $s$- and $p$-wave $\ensuremath{\pi}$-nucleon amplitudes. The persistence of this feature in these nuclei indicates a strong suppression of the net effect of the medium and constrains the interplay of the medium effects.