Observation of a unitary cusp in the thresholdγp→π0preaction

A rigorous multipole analysis of the recent $\gamma p\to {\pi }^{0}p$ 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 $\gamma p\to {\pi }^{+}n\to {\pi }^{0}p$ 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.