Conditions on the pion electroproduction and photoproduction form factors in the soft-pion limit

Using the Bernard-Kaiser-Lee-Meissner (BKLM) parametrization of the hadronic vertex gamma*N -> pi N', with gamma* being a virtual photon, we analyze the restrictions imposed on the form factors (FF) in the limit at which the pion three-momentum and mass go to zero, which is known as the soft-pion limit. We obtain information about the normalization of the FF in this limit using current algebra methods and the hypothesis of partially conserved axial-vector current (PCAC) which is a way to implement chiral symmetry. We relate these FF with those of the (WN)-N-+/- -> N' weak interaction. The present work provides a useful tool to decide whether a certain model is better than another, checking whether the corresponding FF satisfy the conditions imposed in the soft-pion limit, which are model independent.