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 cusp (Wigner cusp), long predicted but never before observed. The analysis is based on a formulation of the Compton cross section employing convenient semiphenomenological expressions for the four dominant multipole amplitudes, designed to respect the low energy theorem and other constraints. Unitarity anomalies have now been detected in both the elastic (Compton) and inelastic (${\mathrm{\pi }}^0$p) channels of the γ+p reaction.