UNITARITY CUSP IN PROTON COMPTON-SCATTERING

Recent proton Compton scattering data from Saskatoon spanning the pion threshold region are shown to reflect the presence of a unitarity cus p (Wiper cusp), long predicted but never before observed. The analysis is based on a formulation of the Compton cross section employing conv enient semiphenomenological expressions for the four dominant multipol e amplitudes, designed to respect the low energy theorem and other con straints. Unitarity anomalies have now been detected in both the elast ic (Compton) and inelastic (pi0p) channels of the gamma + p reaction.