Exclusive Production of Vector Mesons in Qcd

We compare results of our studies [1,2] on vector meson production within the QCD factorization at next-to-leading order with HERA data. 1 Introduction The exclusive vector meson V electro-and photo-productions in photon-proton collisions , γ p → V p, are the subject of HERA collider experiments. The primary motivation for the interests in these processes, both for the light ρ−meson electro-production and for the heavy J/Ψ− or eventually Υ−meson photoproduction, is that they can constrain gluon density in a proton. To achieve this aim it is necessary to minimalize the theoretical uncertainties involved in determination of the coefficient functions (CF), i.e. the perturbatively calculable hard part of the scattering amplitudes. The first step in this direction consists in going beyond the leading Born approximation (LO) and requires calculation of the scattering amplitudes at the next-to-leading order (NLO). Recently such studies were completed both, for the processes with light ρ−meson [1] and for those with heavy vector meson [2]. The theoretical framework of these researches was the QCD factorization which permits to write the scattering amplitude in a form of the convolution (in fractions of longitudinal momenta) of the perturbatively calculable CF with nonperturba-tive distribution amplitude (DA) of produced meson V and the generalized parton distributions (GPDs) in a target proton. We refer the reader to Refs. [1,2] for review of the literature on the subject, for details of derivation and complete set of results. Below we present a first comparison of these new results with HERA data. Let us remind the main source of theoretical uncertainties of our approach. The QCD factorization involves the dependence of the scattering amplitude on two, a priori independent, scales: the factorization scale µ F and the renormalization scale µ R. The dependence of the LO scattering amplitudes on these scales is usually quite strong. By taking into account NLO terms of the amplitudes one hopes to obtain a more consistent description in which the dependence of the amplitudes on µ F and µ R is smoother than in the LO case and that the NLO corrections are small in comparison with LO terms. 2 Electroproduction of ρ−meson