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Deeply virtual electro-production of vector mesons and spin effects.pdf

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Deeply virtual electro-production of vector mesons and spin effects

a r X i v : h e p - p h / 0 6 1 0 0 4 9 v 1 4 O c t 2 0 0 6 Deeply virtual electro-production of vector mesons and spin effects S.V.Goloskokov Bogoliubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna, Russia We study light vector meson photoproduction at small x on the basis of the generalized parton distribution (GPD). Our results on the cross section and spin density matrix elements (SDME) are in fair agreement with DESY experiments. PACS : 12.38.Bx, 13.60.Hb Key words: Diffraction, spin, parton distributions, vector meson production 1 Introduction Vector meson leptoproduction at large energies [1] is one of the important pro- cesses which is sensitive to the GPDs. At small x-Bjorken (x) the process factorizes [2] into a hard meson leptoproduction off gluons and GPD (Fig.1). p = p 2 p0 = p + 2 k = k 2 k0 = k + 2 q q0 = q Fig. 1. The handbag diagram for the meson electroproduction off proton. The generally used standard collinear approximation (SCA) leads to some essen- tial problems. The cross section for longitudinally polarized virtual photons which predominates for the large photon virtualities exceeds the data by a large factor if calculated in the SCA [3]. Only the amplitude with a longitudinally polarized virtual photon and vector meson (LL) amplitude can be calculated in the SCA. The transition amplitudes for transversally polarized photons γ? ⊥ → V⊥ (TT) and γ? ⊥ → VL (TL) exhibit in the SCA the infrared singularities [4] which breakdown factorization. Here we use the modified perturbative approach (MPA) [5] which includes the quark transverse degrees of freedom accompanied by Sudakov suppressions. In the MPA one can solve all mentioned problems. Transverse quark degrees of freedom and Sudakov factors suppress the contribution from the end-point region in the LL amplitude. The transverse quark momentum regularizes the end-point singularities Czechoslovak Journal of Physics, Vol. 56 (2006), Suppl. A A1 S.