Elastic Photoproduction of J/psi Vector Mesons at high Photon-Proton Centre-of-Mass Energy at the H1 Experiment at HERA

The elastic photoproduction of J/psi vector mesons at large values
of the photon-proton centre-of-mass energy
135GeV=180.6GeV)=(5.1+-0.3+-0.2)GeV^-2c^-2,
b(=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.