Estimates for electroproduction of pseudoscalar mesons at small values of skewness are presented. Cross sections and asymmetries for these processes are calculated within the handbag approach which is based on factorization in hard parton subprocesses and soft generalized parton distributions (GPDs). The latter are constructed from double distributions. Transversity GPDs are taken into account; they are accompanied by twist-3 meson wave functions. For most pseudoscalarmeson channels a combination of H T and E T plays a particularly prominent role. This combination of GPDs which we constrain by moments obtained from lattice QCD, leads with the exception of the π + and η ′ channels, to large transverse cross sections.

Electroproduction of light vector mesons is analyzed on the basis of handbag factorization. The required generalized parton distributions are constructed from the CTEQ6 parton distributions with the help of double distributions. The partonic subprocesses are calculated within the modified perturbative approach. The present work extends our previous analysis of the longitudinal cross section to the transverse one and other observables related to both the corresponding amplitudes. Our results are compared to recent experimental findings in detail.

We analyze electroproduction of light vector mesons (V = ρ, φ) at small Bjorkenx in an approach that includes the gluonic generalized parton distributions and a partonic subprocess, γg → (qq)g, qq → V . The subprocess is calculated to lowest order of perturbative QCD taking into account the transverse momenta of the quark and antiquark as well as Sudakov suppressions. Our approach allows to investigate the transition amplitudes for all kind of polarized virtual photons and polarized vector mesons. Modelling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal and transverse cross sections at large photon virtualities as well as the spin density matrix elements for the vector mesons. Our results are in fair agreement with the findings of recent experiments performed at HERA.

We analyze electroproduction of light vector mesons (V = ρ, φ and ω) at small Bjorken-x in the handbag approach in which the process factorizes into general parton distributions and partonic subprocesses. The latter are calculated in the modified perturbative approach where the transverse momenta of the quark and antiquark forming the vector meson are retained and Sudakov suppressions are taken into account. Modeling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal cross sections at large photon virtualities. The results are in fair agreement with the findings of recent experiments performed at HERA and HERMES. 1

Hard exclusive π + electroproduction is investigated within the handbag approach. The prominent role of the pion-pole contribution is demonstrated. It is also shown that the experimental data require a twist-3 effect which ensues from the helicity-flip generalized parton distribution H T and the twist-3 pion wave function. The results calculated from this handbag approach are compared in detail with the experimental data on cross sections and spin asymmetries measured with a polarized target. It is also commented on consequences of this approach for exclusive π 0 and vector-meson electroproduction. 1