ρ0shape in photoproduction

The theoretical and practical difficulties involved in extracting ${\rho }^0$-photoproduction cross sections from measured dipion mass distributions are discussed. To circumvent the theoretical ambiguities, a standard definition of the ${\rho }^0$ cross section is suggested. The definition requires that the mass and width of the ${\rho }^0$ be externally specified; however, we have also attempted to analyze the data to determine these parameters. Because of the theoretical ambiguities, it is not feasible to determine the mass and width from individual curves, and even a simultaneous fit of all the curves gives results which depend on the assumed fitting function. Thus the width has a theoretical ambiguity of order 15 MeV in addition to any statistical uncertainty.