Photoproduction of J/ψ mesons at medium and low elasticities at HERA

An analysis of the photoproduction (Q2 < 1 GeV2) of J/ψ mesons at medium and, for the first time, at low elasticities z is presented. The muonic decay channel is used to select the J/ψ mesons. The data have been collected with the H1 detector at the electron-proton collider HERA and correspond to an integrated luminosity of 40 pb-1. In the medium elasticity region the total photoproduction cross section as a function of the photon-proton centre-of-mass energy Wγp and the single differential cross sections as functions of the elasticity z, the transverse momentum squared of the J/ψ meson pt,ψ2 and the fractional gluon momentum inside the proton xg are extracted. The analysis covers the kinematic range 0.3 < z < 0.9, 60 < Wγp < 180 GeV and pt,ψ2 > 1 GeV2. The results are in agreement with previous HERA measurements where such measurements exist. Theoretical predictions in the next-to-leading order Colour Singlet Model can describe the data. The Wγp dependence favours a gluon density in the proton which rises slowly towards small xg, e.g. MRS(A'). The double differential cross section in pθ,ψ2 and z is extracted and the decay angle distributions of the J/ψ mesons in two elasticity regions are studied. No evidence for significant colour octet contributions is found in any of the distributions studied. (orig.)