Elastic photoproduction of ϱ0 mesons at HERA

The cross section for the elastic photoproduction of ϱ⁰ mesons ( γp → ϱ⁰p) has been measured with the H1 detector at HERA for two average photon-proton centre-of-mass energies of 55 and 187 GeV. The lower energy point was measured by observing directly the ϱ⁰ decay giving a cross section of 9.1 ± 0.9 (stat.) ± 2.5 (syst.) μb. The logarithmic slope parameter of the differential cross section, d σ/d t, is found to be 10.9 ± 2.4 (stat.) ± 1.1 (syst.) GeV ^-2. The ϱ⁰ 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 ± 0.8 (stat.) ± 2.4 (syst.) μb.