Elastic J/ψ production at HERA

In this thesis an analysis of diffractive J/ψ photoproduction and electroproduction in epcollisions is presented. The data were collected with the H1 detector and correspond to an integrated luminosity of 54.79pb−1. The kinematic range covers the region of 40GeV < W < 160GeV and −t < 1.2GeV, where W is the centre-of-mass energy in the photonproton system and −t is the squared four-momentum transfer at the proton vertex. The photon virtuality is Q < 1GeV for J/ψ photoproduction and 2GeV < Q < 80GeV in the region of deep inelastic scattering. Elastic photon-proton cross sections are derived as a function of W , −t and Q in both regions. Also the effect of shrinkage i.e. the change of the t dependence withW , is studied. For this purpose the t dependence of the cross section is measured as a function of W and vice versa. For the first time at H1 shrinkage is also measured for J/ψ electroproduction. All results in both kinematic regions are compared with each other, with previous results from H1 and ZEUS as well as with predictions from different theoretical models. The W dependence of the elastic J/ψ photoproduction cross section, parameterised as W , is measured as δ = 0.71 ± 0.04 ± 0.07. No Q dependence of δ is observed within errors. In the region of deep inelastic scattering δ = 0.78 ± 0.15 ± 0.09 is found. The t dependence of the differential J/ψ cross section is well described by e yielding a value of b = 4.57± 0.07± 0.14GeV−2 for J/ψ photoproduction and b = 4.10± 0.16± 0.20GeV−2 for J/ψ electroproduction. The analysis of the W dependence of the slope parameter b yields an effective Pomeron trajectory slope of α′ = 0.177± 0.034± 0.017GeV−2 for J/ψ photoproduction. For J/ψ electroproduction α′ = 0.018±0.103±0.064GeV−2 is derived, compatible with no shrinkage. The helicity structure of diffractive J/ψ production is analysed. Differential photonproton cross sections are calculated as a function of three different angles which describe the process. The results are compared to previous measurements and the assumption of s-channel helicity conservation. For J/ψ photoproduction the assumption is confirmed, while for deep inelastic scattering small deviations are visible.