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

Abstract

Giant resonances were observed in (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) double charge exchange (at ${T}_{\ensuremath{\pi}}$=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 ${A}^{\mathrm{\ensuremath{-}}1/3}$ 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.