Double giant dipole resonance in the (π−,π+) reaction

The double isovector giant dipole state has been observed in the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{+}$)Δ${\mathit{T}}_{\mathit{x}}$=+2 double-charge-exchange reaction on ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$, and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$. The resonances observed in the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{+}$) reaction are closely related via Coulomb displacement energy and isospin symmetry to the resonances measured in the inverse (${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^{\mathrm{-}}$) reaction on the same nuclei. The new observations provide a direct determination of the upper isospin component of the double giant dipole state, which is generally very weak in the (${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^{\mathrm{-}}$) reaction. The comparison between the double dipole in the two double-charge-exchange modes gives valuable information on the isospin splitting and the Pauli-blocking effects for isotensor transitions.