Diffractive Photoproduction of Rho Mesons with Large Momentum Transfer at HERA

The diffractive photoproduction of rho mesons, ep -> e rho Y, with large momentum transfer squared at the proton vertex, vertical bar t vertical bar, 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 GeV2 and the mass My of the proton remnant is below 5 GeV. The t dependence of the cross section is measured for the range 1.5 <vertical bar t vertical bar <10.0 GeV2 and is well described by a power law, d sigma/d vertical bar t vertical bar proportional to vertical bar t vertical bar(-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 rho 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. (c) 2006 Elsevier B.V. All rights reserved.