Isospin symmetry violation in mirror E1 transitions: Coherent contributions from the giant isovector monopole resonance in the ^{67}As-^{67}Se doublet

The assumption of an exact isospin symmetry would imply equal strengths for mirror E1 transitions (at least, in the long-wavelength limit). Actually, large violations of this symmetry rule have been indicated by a number of experimental results, the last of which is the 67As-67Se doublet investigated at GAMMASPHERE. Here, we examine in detail various possible origins of the observed asymmetry. The coherent effect of Coulomb-induced mixing with the high-lying giant isovector monopole resonance is proposed as the most probable process to produce a large asymmetry in the E1 transitions, with comparatively small effect on the other properties of the parent and daughter levels.