A DISPERSION THEORETICAL APPROACH TO THE THRESHOLD AMPLITUDES OF PION-PHOTOPRODUCTION

We give predictions for the partial wave amplitudes of pion photoprodu ction near threshold by means of dispersion relations at fixed t. The free parameters of this approach are determined by a fit to experiment al data in the energy range 160 MeV less than or equal to E-gamma less than or equal to 420 MeV. The observables near threshold are found to be rather sensitive to the amplitudes in the resonance region, in par ticular to the Delta (1232) and N(1440). We obtain a good agreement w ith the existing threshold data for both charged and neutral pion prod uction. Our predictions also agree well with the results of chiral per turbation theory, except for neutral pion production off the neutron. (C) 1997 Elsevier Science B.V.