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Register a new userHard exclusive photoproduction of charmed mesons
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{We investigate the photoproduction process $ p gamma rightarrow Lambda_{c}^{+} overline{D^{0}}$ within the handbag approach, which we assume to be the dominant mechanism at energies well above the production threshold and in the forward scattering hemisphere.
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We calculate the leading-order perturbative contribution to $gamma p to M_{V} p$, with $M_V$ being a $Phi$ or $J/Psi$ meson, in the kinematic region of large energy and scattering angle.
We report on an attempt to describe hard exclusive photoproduction of $J/Psi$ mesons, i.e. the reaction $gamma p to J/Psi p$, by means of a modified version of the hard-scattering approach, in which the proton is treated as a quark-diquark rather than a three-quark system. In order to improve the applicability of the model at momentum transfers of only a few GeV we take into account constituent-mass effects in the calculation of the perturbative scattering amplitude. With a standard $J/Psi$-meson distribution amplitude and diquark-model parameters adopted from preceding investigations of other photon-induced reactions our predictions for differential cross sections overestimate the naive extrapolation of the low-momentum transfer ZEUS data. Our results, however, reveal the importance of taking into account the charm-quark mass.
In this talk various spin effects in hard exclusive electroproduction of mesons are briefly reviewed. The data are discussed in the light of recent theoretical calculations within the frame work of the handbag approach.
Recent results obtained at HERA on deeply virtual Compton scattering and exclusive vector meson production are reviewed, with the emphasis on the transition from soft to hard diffraction and on spin dynamics.
It is reported on an analysis of electroproduction of light mesons at small Bjorken-x within the handbag approach. The partonic subprocesses, meson electroproduction off quarks or gluons, are calculated within the modified perturbative approach in which quark transverse momenta are retained. The soft hadronic matrix elements, generalized parton distributions, are constructed by means of double distributions. The constraints from parton distributions and sum rules are taken into account. Various moments of these generalized parton distributions are compared to recent results from lattice gauge theories.
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