Elastic and Inelastic Photoproduction of \jpsi Mesons at HERA

TL;DR

This paper reports on the measurement of J/psi meson production in electron-proton collisions at HERA, analyzing elastic, inelastic, and diffractive processes, and comparing results with QCD predictions.

Contribution

It provides new experimental data on J/psi production mechanisms, including elastic, inelastic, and diffractive channels, and tests QCD models against these measurements.

Findings

Elastic J/psi cross section increases with energy

Diffractive J/psi production is comparable to elastic production

Agreement with NLO QCD in inelastic J/psi production

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 $\gamma p$ cross section for elastic \jpsi production is observed to increase strongly with the \cm energy. The cross section for diffractive $J/\psi$ 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 $\gamma p$ 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.