Elastic and Inelastic Photoproduction of \jpsi Mesons at HERA

Abstract

Results on \jpsi production in $e p$ interactionsin the H1 experiment at HERA are presented. The \jpsi mesons are produced by almost real photons ($Q^2\approx 0$) and detected via their leptonic decays. The data have been taken in 1994 and correspond to an integrated luminosity of $2.7 {pb}^{-1}$. The $γp$ cross section for elastic \jpsi production is observed to increase strongly with the \cm energy. The cross section for diffractive $J/ψ$ production with proton dissociation is found to be of similar magnitude as the elastic cross section. Distributions of transverse momentum and decay angle are studied and found to be in accord with a diffractive production mechanism. For inelastic \jpsi production the total $γp$ cross section, the distribution of transverse momenta, and the elasticity of the \jpsi are compared to NLO QCD calculations in a colour singlet model and agreement is found. Diffractive \psiprime production has been observed and a first estimate of the ratio to \jpsi production in the HERA energy regime is given.