We analyze in detail the size of next-to-leading order corrections to hard exclusive meson production within the collinear factorization approach. Corrections to the cross section are found to be huge at small x B and substantial in typical fixed-target kinematics. With the models we take for nucleon helicity-flip distributions, the transverse target polarization asymmetry in vector meson production is strongly affected by radiative corrections, except at large x B . Its overall size is very small for ρ production but can be large in the ω channel.

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We study the evolution behavior of generalized parton distributions at small longitudinal momentum fraction. Particular attention is paid to the ratio of a generalized parton distribution and its forward limit, to the mixing between quarks and gluons, and to the dependence on the squared momentum transfer t.

We investigate the role of exclusive channels in semi-inclusive electroproduction of pions and kaons. Using the QCD factorization theorem for hard exclusive processes we evaluate the cross sections for exclusive pseudoscalar and vector meson production in terms of generalized parton distributions and meson distribution amplitudes. We investigate the uncertainties arising from the modeling of the nonperturbative input quantities. Combining these results with available experimental data, we compare the cross sections for exclusive channels to that obtained from quark fragmentation in semi-inclusive deep inelastic scattering. We find that ρ 0 production is the only exclusive channel with significant contributions to semiinclusive pion production at large z and moderate Q 2 . The corresponding contribution to kaon production from the decay of exclusively produced φ and K * is rather small.

We report on numerical studies of the NLO corrections to exclusive meson electroproduction, both in collider and fixed-target kinematics. Corrections are found to be huge at small xB and sizeable at intermediate or large xB. Motivation and general frameworkGeneralized parton distributions (GPDs) are a versatile tool to quantify important aspects of hadron structure in QCD. They contain unique information on the spatial distribution of partons [1] and on the orbital angular momentum they carry in the proton [2]. The theoretically cleanest process where GPDs can be studied is deeply virtual Compton scattering (similar to inclusive DIS, which plays a dominant role in constraining the usual parton densities). Hard exclusive meson production is harder to describe quantitatively, but it provides opportunities to obtain important complementary constraints. In particular, vector meson production is more directly sensitive to the gluon distributions, which enter the Compton amplitude only at next-to-leading (NLO) order in α s . Together with a wealth of high-quality data [3], this warrants efforts to bring meson production under theoretical control as much as possible.In the present contribution [4] we investigate exclusive ρ production (γ * p → ρp) using collinear factorization, which is applicable in the limit of large photon virtuality Q 2 at fixed Bjorken variable x B and fixed invariant momentum transfer t to the proton [5]. In practical terms, this means that the description is restricted to sufficiently large Q 2 but can be used for both small and large x B , thus providing a common framework for analyzing both collider and fixed-target data. The process amplitude can then be expressed in terms of GPDs for the proton, the qq distribution amplitude for the ρ, and hard-scattering kernels. The kernels are known to NLO, i.e. to order α 2 s [6]. The requirement of "sufficiently large" Q 2 is demanding for meson production. Contributions that are formally suppressed by powers of 1/Q 2 cannot be calculated in a completely systematic way, but the estimates [7,8,9] agree that for Q 2 of several GeV 2 the effect of the transverse quark momentum inside the meson cannot be neglected in the hard-scattering subprocess, as it is done in the collinear approximation. This effect can be incorporated in the modified hard-scattering picture [7,8], in color dipole models [9], or in the MRT approach [10]. Unfortunately, the calculation of α s corrections remains not only a technical but even a conceptual challenge in these approaches, so that the perturbative stability of their results cannot be investigated at present. One strategy in this situation is to study the NLO corrections in the collinear factorization framework, identifying kinematical regions where they are moderate or small. There one may use formulations incorporating power corrections from transverse quark momentum with greater confidence. This is the aim of the present contribution.In the following we show results for the convolution of the unpolarized quark and gluon G...