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

We report the observation of double isovector giant dipole resonances in ({pi}{sup +},{pi}{sup {minus}}) and, for the first time, also in ({pi}{sup {minus}},{pi}{sup +}) double-charge-exchange reactions at {ital T}{sub {pi}}=292 MeV. The resonances have been observed on nine targets covering a wide range of mass at {ital Q} values around {approx}{minus}50 MeV in ({pi}{sup +},{pi}{sup {minus}}), but at significantly lower energy ({ital Q}={minus}33.8 MeV) in the inverse reaction on {sup 56}Fe. The centroid energies in both reactions are very close to the energies at which the {Delta}{ital T}{sub {ital z}}={plus minus}2 double giant dipole ({ital J}{sup {pi}}=0{sup +},2{sup +},{Delta}{ital T}=2) states are expected to appear. Angular distributions have been measured for the ({pi}{sup +},{pi}{sup {minus}}) resonances on {sup 40}Ca,{sup 56}, Fe, and {sup 93}Nb and each is observed to have a quadrupole shape. Based on their energies, angular distributions, cross sections, and the comparison between the ({pi}{sup +},{pi}{sup {minus}}) and ({pi}{sup {minus}},{pi}{sup +}) data on {sup 56}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 more » of all nuclei. « less