E0 suppression in pion photoproduction on 13C

Abstract Recently measured anomalously low cross sections for 13 C(γ, π − ) 13 N at low energy and θ π lab = 90° 0761 have been analyzed in a DWIA calculation. It has been found that the EO contribution alone is able to explain the data, so that the M1 cross section is expected to vanish. Using constraints from recent magnetic electron scattering, an explanation is possible by assuming a significantly lower reduced density matrix element for spin-flip isovector transitions with angular momentum L = 2 than predicted by Cohen and Kurath.