Observation of a unitary cusp in the threshold gamma p->pi(0)p reaction

A rigorous multipole analysis of the recent gamma p-->pi(0)p cross-section measurement is presented. The data were taken using the photon spectrometer TAPS at the tagged photon beam of the Mainz microtron. The s and p wave multipoles were extracted using minimal model assumptions. The predicted unitary cusp for the s-wave multipole E(0+) due to the two step gamma p-->pi(+)n-->pi(0)p reaction was observed. The results are consistent with one-loop chiral perturbation theory calculations for which three low-energy constants have been determined by a fit to the data. The uncertainties in the analysis and the need for polarization observables are discussed.