Elastic Photoproduction of $\rho^{0}$ Mesons at HERA
  The cross section for the elastic photoproduction of \r0 mesons ($\gamma p \to \rho^0 p$) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. TheFcenterline lower energy point was measured by observing directly the $\rho^{0}$ decay giving a cross section of $9.1\pm 0.9 (\stat)\pm 2.5 (\syst) \mu$b. The logarithmic slope parameter of the differential cross section, ${\rm d}\sigma/{\rm d}t$, is found to be $10.9 \pm 2.4 (\stat) \pm 1.1 (\syst) $GeV$^{-2}$. The \r0 decay polar angular distribution is found to be consistent with s-channel helicity conservation. The higher energy cross section was determined from analysis of the lower part of the hadronic invariant mass spectrum of diffractive photoproduction and found to be $13.6\pm 0.8 (\stat)\pm 2.4 (\syst) \mu$b. 
S. Aid, et al. (H1 Collaboration)