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. 
A. M. Bernstein, E. Shuster, R. Beck, M. Fuchs, B. Krusche, H. Merkel,   H. Stroher