MESON-BARYON-BARYON VERTEX FUNCTION AND THE WARD-TAKAHASHI IDENTITY

Ohta proposed a solution for the well-known difficulty of satisfying t he Ward-Takahashi identity for a photo-meson-baryon-baryon amplitude ( gamma MBB) when a dressed meson-baryon-baryon (MBB) vertex function is present. He obtained a form for the gamma MBB amplitude which contain ed, in addition to the usual pole terms, longitudinal seagull terms wh ich were determined entirely by the MBB vertex function. He arrived at his result by using a Lagrangian which yields the MBB vertex function at tree level. We show that such a Lagrangian can be neither Hermitia n nor charge conjugation invariant. We have been able to reproduce Oht a's result for the gamma MBB amplitude using the Ward-Takahashi identi ty and no other assumption, dynamical or otherwise, and the most gener al form for the MBB and gamma MBB vertices. However, contrary to Ohta' s finding, we find that the seagull terms are not robust. The seagull terms extracted from the gamma MBB vertex occur unchanged in tree grap hs, such as in an exchange current amplitude. But the seagull terms wh ich appear in a loop graph, as in the calculation of an electromagneti c form factor, are, in general, different. The whole procedure says no thing about the transverse part of the (gamma MBB) vertex and its cont ributions to the amplitudes in question.