Spinor Ladder Models and the Asymptotic Behavior of Neutral Pion Photoproduction Amplitudes

The asymptotic behaviors of nucleon-antinucleon scattering and n°-photoproduction off a proton are calculated in perturbative ladder models whose sides are both spinor lines. Spins of the particles are treated correctly. Non-integer fixed branch points of the square-root type in complex angular momentum plane are found to dominate the amplitudes when s goes to minus infinity with t fixed. This disproves the previous conjecture that the right signature fixed pole at J=O may dominate the n°-photoproduction amplitude in the spinor ladder model, and seems to make the fixed pole unsatisfactory in explaining the experimental behavior s2dff / dt= constant.