Abstract

The elastic photoproduction of J/psi vector mesons at large values of the photon-proton centre-of-mass energy 135GeV<W_{gamma p}<305GeV is studied. The elastic photoproduction cross section sigma(W_{gamma p}) is measured as a function of W_{gamma p}. The differential cross section dsigma/dt is determined in the range |t|<=1.2GeV^2/c^2. This analysis extends the measurements of sigma(W_{gamma p}) and dsigma/dt using H1 data up to highest W_{gamma p} near the kinematic limit given by the parameters of the accelerator of approx. 318GeV. Data for two different event topologies were collected with the H1 detector at the ep-collider HERA during the years 1999 and 2000 resulting in an integrated luminosity of 30.26pb^-1 and 26.90pb^-1 for the analysed data. The level 2 neural network trigger and the backward silicon tracker are important components of the H1 experiment which make this analysis possible. From the W_{gamma p} dependence of the elastic cross section sigma(W_{gamma p})~W_{gamma p}^delta the parameter delta is extracted to be delta=1.17+-0.11+-0.16 for the high W_{gamma p} results of this analysis alone and delta=0.76+-0.03+-0.04 including data of the complete kinematic range of the H1 experiment. The exponential slope b of the differential cross section dsigma/dt~e^{-b|t|} is determined in the high W_{gamma p} regime: b(<W_{gamma p}>=180.6GeV)=(5.1+-0.3+-0.2)GeV^-2c^-2, b(<W_{gamma p}>=250.7GeV)=(5.4+-0.4+-0.2)GeV^-2c^-2. The intercept alpha_0 and slope alpha' of an effective pomeron trajectory are determined. The W_{gamma p} dependence of the b-slope gives alpha'_b=(0.17+-0.04+-0.01)GeV^-2c^-2. The W_{gamma p} dependence of the differential cross section dsigma/dt yields both parameters: alpha_0=1.21+-0.01+-0.01 and alpha'=(0.14+-0.03+-0.03)GeV^-2c^-2. The two determined slope parameters alpha'_b and alpha' are compatible within errors. Different theoretical approaches are compared to the data. The single pomeron model based on Regge theory does not describe the strong rise of sigma(W_{gamma p}) as a function of W_{gamma p}. The two pomeron model is able to describe the W_{gamma p} dependence of the cross section sigma(W_{gamma p}) within the kinematic regime of HERA, but is not able to describe the combined results of HERA and of fixed target experiments at low values of W_{gamma p}. QCD predictions using different parameterisations of the gluon distribution in the proton are also compared to the measured cross section sigma(W_{gamma p}). The predictions show a strong dependence on the gluon distribution, since the gluon distribution enters the cross section quadratically. The comparison to the measured cross section sigma(W_{gamma p}) is therefore a sensitive test for the gluon distributions, which are determined in an indirect way from inclusive measurements. While some of these analyses result in gluon distributions consistent with the J/psi measurements presented here, others can definitely be excluded.