GAUGE-INVARIANT TREE-LEVEL PHOTOPRODUCTION AMPLITUDES WITH FORM-FACTORS

We show how the gauge-invariance formulation given by Haberzettl is im plemented in practice for photoproduction amplitudes at tree level wit h form factors describing composite nucleons. We demonstrate that, in contrast to Ohta's gauge-invariance prescription, this formalism allow s electric current contributions to be multiplied by a form factor, i. e., it does not require that they be treated like bare currents. While different in detail, this nevertheless lends support to previous ad h oc approaches which multiply the Born amplitudes by an overall form fa ctor. Numerical results for kaon photoproduction off the nucleon are g iven. They show that the gauge procedure by Haberzettl leads to much i mproved chi(2) values compared to Ohta's prescription.