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

The double isovector giant dipole state has been observed in the (π^- ,π⁺)ΔT_x=+2 double-charge-exchange reaction on ¹³C, ²⁷Al, ⁴⁰Ca, ⁵⁶Fe, ⁵⁹Co, and ⁹³Nb. The resonances observed in the (π^- ,π⁺) reaction are closely related via Coulomb displacement energy and isospin symmetry to the resonances measured in the inverse (π⁺,π^-) 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 (π⁺,π^-) 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.