Observation of Isobaric Analog States in Pion Single-Charge-Exchange Reactions

Isobaric-analog-state transitions have been observed throughout the periodic table in the (${\pi }^{+}$,${\pi }^0$) reaction. The forward-angle differential cross sections at $T_{\pi }=98\mathrm{}$ MeV on targets of C${\mathrm{H}}_2$, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{120}{}_{}{}^{}\mathrm{Sn}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ are reported. The $A$ dependence follows $\mathrm{}(N-Z)\mathrm{}{A}^{-\frac{4}{3}}$, which is qualitatively understood in terms of a strong-absorption model. Optical-potential calculations which demonstrate the sensitivity to neutron density distributions and to the isospin dependence of higher-order dynamical effects are discussed.