Observation of double isovector giant dipole resonances from pion double charge exchange

Abstract

We report the observation of double isovector giant dipole resonances in (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) and, for the first time, also in (${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$,${\ensuremath{\pi}}^{+}$) double-charge-exchange reactions at ${T}_{\ensuremath{\pi}}$=292 MeV. The resonances have been observed on nine targets covering a wide range of mass at Q values around \ensuremath{\approxeq}-50 MeV in (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$), but at significantly lower energy (Q=-33.8 MeV) in the inverse reaction on $^{56}\mathrm{Fe}$. The centroid energies in both reactions are very close to the energies at which the \ensuremath{\Delta}${T}_{z}$=\ifmmode\pm\else\textpm\fi{}2 double giant dipole (${J}^{\ensuremath{\pi}}$${=0}^{+}$,${2}^{+}$,\ensuremath{\Delta}T=2) states are expected to appear. Angular distributions have been measured for the (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) resonances on $^{40}\mathrm{Ca}$${,}^{56}$, Fe, and $^{93}\mathrm{Nb}$ and each is observed to have a quadrupole shape. Based on their energies, angular distributions, cross sections, and the comparison between the (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) and (${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$,${\ensuremath{\pi}}^{+}$) data on $^{56}\mathrm{Fe}$, we identify the resonances as double giant dipole states (i.e. states arising from a charge-exchange dipole operator acting twice on the ground-state wave function). The data from the present and recent observations indicate that the newly discovered resonance is a general collective feature of all nuclei.