The diffractive photoproduction of ρ mesons, ep → eρY , with large momentum transfer squared at the proton vertex, |t|, is studied with the H1 detector at HERA using an integrated luminosity of 20.1 pb −1 . The photon-proton centre of mass energy spans the range 75 < W < 95 GeV, the photon virtuality is restricted to Q 2 < 0.01 GeV 2 and the mass M Y of the proton remnant is below 5 GeV. The t dependence of the cross section is measured for the range 1.5 < |t| < 10.0 GeV 2 and is well described

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... by a power law, dσ/d|t| ∝ |t| −n . The spin density matrix elements, which provide information on the helicity structure of the interaction, are extracted using measurements of angular distributions of the ρ decay products. The data indicate a violation of s-channel helicity conservation, with contributions from both single and double helicity-flip being observed. The results are compared to the predictions of perturbative QCD models.