Helicity amplitudes in pion photoproduction

Asymptotic QCD predicts that, as energy increases, the differential cross section for pion photoproduction should depend on the energy squared s as s-7. In addition, the helicity of the nucleon should be conserved. The differential cross section at 90 degrees center-of-mass angle for each of the four pion-production channels is in reasonable agreement with this energy dependence for energies above s= 1.4 GeV2. The question of helicity conservation can be investigated by looking at the four helicity amplitudes that describe the reaction. These amplitudes are calculated by a partial-wave analysis of data for differential cross sections and polarization measurements. The current VPI partial-wave analysis, for energies s< 4.3 GeV2 shows no evidence that the amplitudes vanish