Giant dipole resonances built on isobaric analog states in pion double charge exchange

Abstract

Giant resonances were observed in (${\pi }^{+}$,${\pi }^{\mathrm{-}}$) double charge exchange (at $T_{\pi }$=292 MeV) on ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{80}{}_{}{}^{}\mathrm{Se}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ at excitation energies of 28.5, 34.6, and 43.6 MeV, respectively. An angular distribution for ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ was measured and observed to have a dipole shape. The variation of excitation energy above the double isobaric analog state is consistent with an $A^{\mathrm{-}1/3}$ dependence, as expected for giant dipole resonances built on the isobaric analog states. The ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ angular distribution agrees well with a coupled-channels impulse-approximation calculation in which sequential single charge exchange through the isobaric analog to the giant dipole resonance is evaluated.